Bob,
The P90 is a good solid pistol, but haven't seen any aftermarket parts for it. Guess if you REALLY wanted to hang things on it, you could find a scope base with the proper contour, have the dust cover drilled and tapped to screw it on. You might be happier buying another inexpensive pistol with the rail. Not sure, but I think the Ruger P345 has a rail.
Savage