[Nyororin]

Basically, you`ll need to adopt the child as a US citizen, following regular procedures to do so... And then bring the child into Japan as your adopted child. Which really shouldn`t be that hard. You just have to keep an eye on the age cut off for an adopted child to be applicable for a dependent visa. There is some level of strictness there. I think it is 6 at the time of adoption. A child over that cannot be brought into the country... Or rather, cannot obtain a visa as your dependent.

It should just be a matter of paperwork if the child is under 6. The potential problems would most likely lie with the country you are adopting the child from, and what they require of you.