Can you put a bmx stem on a mountain bike?

Most BMX have sprockets with freewheels as one unit. This means that you need a special puller to grip notches on the freewheel to be able to unscrew the old one. The new one can be started by hand and will tighten down by itself as you pedal. If you have a low-end one with a built-in coaster brake other rules apply and you're probably looking at something with a lockring arrangement.