Possible to be nuclear, yes, even if unlikely at the moment. Possible to hide even a small scale test underground, damn near impossible. And why the fuck would Russia let another country test a nuke within their borders (let's not even mention the seismic signature in a country not known for seismic activity relatively speaking)? Think through your ideas, man...
Again, think through your ideas. Buying a bomb from another country is moronic. Too easily traced, and not to mention Pakistan's loose warheads are stored disassembled, meaning unless Pakistan plans on removing a warhead from a missile (kind of obvious), or going through the pretty complex (read: hard to hide) work of assembling warheads all of sudden. This is also not to mention the rumors of Pakistan already having a nuclear sharing deal with Saudi Arabia.