Yes you can use a Japanese phone in America!

The answer is yes, BUT you must make sure the phone you're purchasing from Japan:

1.) Is Tri-Band 900/1800/1900 make sure it operates on these frequencies. You can pop out your T-Mobile sim card and put it in your Japanese Phone.

2.) Is Unlocked! Make sure the phone from Japan isn't locked to any carrier.

Also make sure you have a wall adapter for the phone's charger, so you can use it in your home here in the US.