JCAB Atpl can be converted from another ICAO Atpl through an airline, but only after you have considerable experience. Anything before that (eg from a FAA CPL) will be very expensive and requires a lot of training in Japan.
I recommend you go to the US, get CPL asap and try to stay there as instructor. You’ll get a visa for that as many have done already. Afterwards you can get into a regional if things stay as they are now, and in the future Japan will be an option for sure.