Would it, though?

For a simple example, the Mongol empire used a meritocratic system of succession, through election by the aristocracy. And that empire lost cohesion when a general named Kublai broke the rules and usurped the usual election process.

A war of succession has three factors - who has a claim, the popular support of each claim, and the military backing of each claim. Appointing a stepson or stepdaughter as successor would matter only in circumstances where that wish is likely to be honored and possible counter-claimants don't have the guts or resources to assassinate or depose the successor.

On a case by case basis, circumstances differ. But a war of succession isn't prevented by a magical mandate of the ruler, it's prevented by a solid base of power without potent conflicting factions. There have been rather smoother transitions of power between dynasties and less smooth ones. But, returning to my example, the proper election of Ariq Boke didn't stop Kublai from marching on his own people.

Expanding on coven's answer, it is important to note that Medieval succession laws evolved over a long time and were relatively formalized. Adoption was not a recognized method of entering ruling house's blood line. Assuming one was in a primogeniture based society, particularly under Salic Law, the claims to the throne really only run as deep as your own paternal line by blood. In addition, more importantly, almost every ruling house had numerous sub-branches that could (and often did) claim the right of inheritance if the main branch should die out without male issue, so there were almost always families out there with claims to the throne.

At best a monarch could attempt to influence which of the noble houses might take over, but generally these situations sort of occured on an ad hoc basis. Very few monarchs accepted that they would have no male issue before they died, and thus the most common situation was a monarch dying without leaving any sort of plan.

In Japan, they did. According to Japan Emerging, edited by Karl Friday, adoptions were quite common among warlords. One man, Uesugi Kenshin had an adopted father. Many warlords would adopt children if they had no male heir or found someone who they thought was more competent than their own children.