I didn't bother reading the rest of the posts, so I don't know if your question has been answered. I don't support what these people do, but they have done a few good things while the government left something to be desired, so I won't look down on them for the choices they have made, I don't know what circumstances made them join.

Anyway, I have read about Koreans being part of certain groups of Yakuza. I can't remember which though, and don't know if these are Koreans born in Japan, but they are for all intents and purposes foreigners according to the Japanese. There are also Yakuza in the US and other places outside Japan, but I can't remember if they allow other ethnicities into their gang. It would probably be better for you to look elsewhere.

I know an American who was asked. Of course he is like 6 foot 8 inches tall and weighs about 300 pounds (all muscle). It is possible, but not likely.