Re: is it possible to change a single to a hum ?
Does it have a scratch plate are the pickups mounted in rings to the top of the guitar?
If the former, it depends on the routing of the guitar. If it has a "swimming pool" route (one big chunk of wood taken out for all three pups), get a new pickguard, mount the desired pickups in it and slap it on the guitar.
If it's a rear-loaded guitar (pickup rings), it would be major surgery to add a humbucker.