#1
This may sound stupid but I want to put a P puckup on my jazz bass in addition to the jazz pickups. Is this possible? If yes how much a luthier charge me on average?

(There is more than enough space for one inbetween the neck J pickup and the neck.)
#2
Its possible, you would just have to make some major mods to the electronics, pickguard, and control plate. You would have to wire up another volume and tone knob, or maybe an on/off switch for it. You would need a new control plate for the extra knobs, a new pickguard with your P-pickup hole. I, personally, would think it too much a hassle to do and would just buy a P-bass or a bass with a P and J pickup in it. If you do end up doing it, have him route it more like the Stu Hamm bass i think it is, with the P pickup between the J pickups.
#3
Hmm, its one of the few J Basses without a pickup or control plate so thats not a porblem, I'll just make one my self, how hward can it be (famous last words). Wouldnt it be possible to rewire all the wickups to a single pickup selector?