It doesn't fit. I tried it.
The US and JP expansion ports, while they share the same pin-out, have a different shape completely. The JP ports are squared while the US ports have slanted edges.
Since they are electrically compatible, Michael is more or less correct in that I'm sure it could be modified to fit, probably involving cutting away plastic, taking PCBs out, etc.
Once you got it to fit, however, your dreams would probably be shattered because the RAU-30 connector doesn't pass power for some reason. I guess in Japan they like to have two AC adapters hooked up all the time because 2 is better than 1.