Could a UAV ever be hacked and piloted?
Not only is it possible to hack a UAV, but it has already happened. In December 2011 a US RQ-170 recon drone landed in Iran after being hijacked by Iran's security services, possibly with Russian help. This was as much poor security implementation on the drone as clever hacking on the part of Iran. 

As for whether it would be possible to take over an armed drone and fire its weapons, it would certainly seem so.