The simple answer is that the "if" you pose is wrong. Pre-modern Japan had an adequate iron supply. In the early 20th century, Japanese domestic iron ore production was about 135,000 tons per year, which, while small compared to the 600,000 tons of ore imported in 1918, was much larger than the amount used by the traditional iron industry, which was producing about 10-15,000 tons of iron and steel per year at the end of the Edo Period (so perhaps 30,000 tons of ore per year).

The Edo Period limit on iron production wasn't the availability of ore, but labour costs and fuel (charcoal). The threat of deforestation - a significant part of which was due to the iron industry - led to environmental protection laws by the Tokugawa Shogunate.

How many soldiers can you equip with armour with 1,000 tons? Assuming a lot of waste in manufacturing, with much of the iron lost as forge scale, 50kg would provide sufficient iron/steel for the armour and weapons of a fully-armoured soldier. So if 1,000 tons per year went into the arms industry, that would be 20,000 fully-armoured soldiers newly equipped per year.

The largest armies of the time were about 150,000 men, and the largest battles had about 200-300,000 men, so it would have taken a few years to make enough armour for them all (not that all of them were fully armoured), but was certainly possible.

Also note that a lot of Japanese armour was rawhide lamellar armour, or a mix of rawhide and iron lamellae, so some of what looks like iron armour is not.