Yes you can and it's perfectly legal.
Just do an ATF Form 1 like you normally would. However,
When it becomes a rifle it must have 5 American made parts to be 922r compliant. If you check out
http://www.hkparts.net, they have 922r kits available for their builds.
Essentially this is an MP5-K, so they make parts for those.
-Linny