There have definitely been a few mutterings here and there about portions of Manchuria that "should" belong to a united Korea, but at the state level, I don't think North Korea ever seriously made expansion a platform outside of the Korean War. NB: In its internal propaganda, North Korea never actually billed the Korean War as an attempt to annex the South. To this day, the government continues to insist that South Korea attacked first and that the conflict was entirely defensive in nature.

Manchuria: To address the Manchuria bit really quick, there is an historical reason for Korean ultra-nationalists' argument that it should be part of greater Korea. A significant chunk of what's now northeastern China and southeastern Siberia was part of Korean tribal territories, the Korean kingdom known as Koguryeo/Goguryeo during the Three Kingdoms period, and Balhae during the North-South States period. All told, this was an era spanning almost a thousand years from 37 BCE to 926 CE. Even though the territory now largely belongs to China, there's still a Korean-speaking minority in the region, to the point where North Korean defectors and refugees who escape over the Yalu River are able to get along without any serious command of Chinese.

To the ultra-nationalists, this is proof that the border got drawn in the wrong place. And there are plenty of national borders around the world that are kinda bullshit, so they'd be in good company. But are they right?

To sociologists, the issue is much less straightforward, because the Korean minority is something like 0.6% of Liaoning province, 4% of Jilin province, and 1% of Heilongjiang province. (Plus a pretty negligible number, possibly even <0.1%, in Inner Mongolia.) So when we say minority, we really do mean minority; if a unified Korean state somehow got its hands on most of Manchuria today, this would be an issue. But somehow I don't see China willingly ceding any portion of this territory.

To historians, the issue is murkier still. The unified Korean state (Goryeo) that succeeded Balhae/Silla in the 10th century never held any portion of Manchuria, and while Joseon (the state that succeeded Goryeo in the late 14th century) expanded its borders to the approximate northern reach of North Korea today, it never held anything beyond that.

So yes, the ultra-nationalists are right that people ethnically and linguistically identifiable as "Koreans" held portions of Manchuria for almost a thousand years, but then they spent the next thousand culturally present but not politically dominant to any serious or lasting degree. This is usually the point at which you get punched in a bar somewhere in Seoul, which is why historians do not talk shop with angry, nationalistic drunks.

Anyway! To the best of my knowledge, the North Korean state has never been in favor of conquest elsewhere. While it's possible that I've missed something, I'm struggling to remember anything I've seen in North Korean propaganda or literature that's seriously argued for the expansion of the North Korean state. If anything, NK's propaganda tends to harp on the Korean race's uniqueness and purity in a harsh world, requiring the protection and guidance of a strong leader and his party. That's fundamentally incompatible with the notion of being a conquering power.

There are likely a few reasons for this:

• North Korea's military weakness: Realistically, North Korea never had the capacity to run an offensive campaign against anything but a weak state with which it shared a land border. (Which more or less describes post-World War II South Korea.) And even then, it really didn't have the capacity to sustain it. In its best years, NK still had difficulty with the agricultural and industrial production and surpluses required for self-sufficiency, much less a war effort. While this is straying into /r/HistoricalWhatIf territory, they may not have been able to hold onto SK permanently in the absence of the U.S./U.N. response during the Korean War. So the notion of invading anywhere else would have been an utter fantasy, especially for something like Japan, which would have required ships and planes for troop/supply transport that NK didn't have and couldn't have produced in quantity.

It's an old saying in the military community that strategy wins battles but logistics wins wars. You can learn a lot about a potential opponent by studying how its military equips, trains, and supplies itself, and whether this is done with an eye toward action elsewhere. NK's military has traditionally been personnel-heavy, but they didn't build a support structure to project power elsewhere, and there have always been severe economic limits on their ability to do so. The Kim regime was not unaware of this, and it was arguably one of the things driving the birth of North Korea's missile program in the 1980s as South Korea's star was on the rise. The Kims wanted an equalizer that, once created, would not be dependent on the country's economic circumstances, troop strength, or supply chain.

• Its dependence on both China and the Soviet Union: NK has only three neighbors: South Korea, China, and Russia. (The Russian border is admittedly tiny -- it's ~12 miles at most.) SK we've already covered, but as to China and the U.S.S.R./Russia? Both could have crushed North Korea (always a deterrent to invasion!), but the situation was more complicated than just that. North Korea was thoroughly dependent on both during the Cold War, not just for its own protection (as NK saw it) but just to keep its own economy running at all. China and the Soviet Union were NK's primary export markets during the Cold War, and they were also the source of a tremendous amount of technological and economic help. The most notable was probably the Soviets' "Friendship Prices" program, which allowed NK to purchase things like oil and gas at 25% of their actual market value. The end of this program arguably started the downward spiral culminating in NK's 1994-1998 famine.

Client states can generally be relied upon not to annex their benefactor's territory, and the benefactor tends to get a weighty vote on what the client state will be permitted to do geopolitically. What do most hegemonic states want with respect to their client states? Stability. If that fails, they generally want their client states not to do stupid destructive bullshit that will put everyone in the region on the road to an unwanted conflict. The Soviets and the Chinese were prepared to pull Kim il-Sung's chestnuts out of the fire during the Korean War -- doing otherwise would have meant having a unified Korean state and American ally sitting on their doorstep -- but they were certainly not going to bankroll or assist NK in adventures elsewhere.

• The influence of history and culture. This is where I feel shakiest -- "culture" can be a very lazy way to argue something -- and I won't spend much time here as a result. However, it could be argued that the Korean peninsula's lack of history with conquest or forced assimilation of other nations/cultures is playing a role too. Korea is quite isolated geographically and is separated from its neighbors on all sides by seas, rivers (the Yalu to the west with China and the Tumen to the east with both China and Russia), and mountains. That's excellent from a defensive perspective but not great if you'd rather treat your military like an outreach program. Somewhat tellingly, Goryeo's final borders in the late 14th century are reasonably close to the modern Korean states of today, and Joseon's were almost identical as of 1433. The "hermit kingdom" thing was a real phenomenon, and remains so in North Korea.

So from the North Korean perspective, there was no history of conquest that Korea had "lost" during the Japanese occupation, and the state didn't need to expand its borders to be legitimate in the eyes of its own people.

As a North Korea specialist, I've never heard of it. If North Korea had successfully reunified the peninsula, it's likely the regime would have continued pursuing it's harshly anti-Japanese propaganda strings to the point where there were small-scale naval skirmishes with Japan. But I don't think any North Korea ruler ever had any design to take over Japanese mainland territory.