I couldn't do it for Europe, so I doubt you could do it for Japan (I didn't think they'd let you import a car that was less than 30 years old). I was told I'd need a whole new nave system, not just a disc.
Being military, you really aren't "importing" the car. We can get away with a few more things than most. You can only bring one vehicle with you, which is why a lot of military families end up buying a cheap "domestic" car in whatever country they live in.
When I PCS'd to Misawa, I was told I couldn't bring a vehicle unless it was 30+ years old (or it was prohibitively expensive, I don't remember).
It's different here in the UK, I was able to bring my Charger over at no cost (other than my nav disc being stolen). That's when I started calling around to find out if I could just put a European nav disc in. Everyone said no. Basically, the whole nav unit needs to be the European version for the European disc to work. I hope it's different in Japan.