Relax Bic, what you are talking about IS doable but you would be creating what the National Firearms Act calls a Short Barreled rifle" if you were going to actually do it you would need to pay a $200 tax to ATF and get your paper work back BEFORE you assembled it. It's when you take a RIFLE receiver and cut the barrel down to pistol length that they flat don't allow.
But what you would be doing is making a PISTOL longer, so it's just like buying it in the first place, follow the law and you would have no trouble.
As far as the actual mechanics of it AR should just screw in like the buffer tube, FAL I don't know, I'm not familiar with them.