Here is what the rifle looks like:

Also, here is the bullet for it. For size comparision, there is an American Quarter to the left (and its an Oregon Quarter ;D), in the middle is the .17HRM bullet, and to the right is a standard .22 bullet.

Trust me, if there are P-90's in private hands in the US then they do not own them legally. FN is producing them solely for the military and law enforcement markets.
My point is, there are no laws that outlaw such a firearm-therefore, you CAN own them. If the producer is selling them exclusivily to military groups-then there probably aren't any legal ones for that reason. But, there is no law outlawing the ownership of these weapons. What it comes down to is if anyone has successifully imported one legally and from what I gather-they probably haven't.