A conversion from .32 to .380 might be possible, but not to 9mm or 7.62x25, IMHO. I do know that the factory 9mm is actually larger in size. I can confirm that the rate reducer is not installed in many semi autos. This is not just for FA, but to slow the bolt going back and forth. In a sense it is for delay and acts like a buffer. So how could it be made in 9mm? I am guessing it would work like the slowfire MAC-11s. I have seen CAR stocks at the rear and the CAR buffer adds mass to the bolt via a hole at the back of the receiver.
Now I am not suggesting something as tacky as a CAR stock! But think about the FAL stock and the spring it has in it. There is a version of the Skorpion with a wooden stock. Maybe a steel buffer and heavy spring could be placed inside to allow for delay? As for magazines, I don't think a 9mm Parabellum will fit in given the bullet shape.