Is it possible to power a piezo from a guitar that already has active electronics and magnetic pups?
yeah you can.....
Im not sure, but you may also have to add another 9v battery, i doubt it though. Some smarty electro person will come help you out
I'm sure it would be possible as long as your pre-amp isn't inside the pickup (EMG like pickups)... if the pre-amp is outside the pickups, all you would need to do is make sure that the pre-amp comes after the pickup selector in the circuit, and you're golden.