Besides a J pickup? If there is a cool P bass that I would like to turn into a P/J, is this like a do-myself kind of thing? - Get pickup / Mount it - Drill hole for wires - Get blend knob, or switch, - Move Jack to side.... Missing anything?
Some drilling, some routing, some soldering. Possibly some finish touch up depending on how well the routing and drilling went. There are surface mount pickups but I have not seen any Jazz style pickups that are meant to be surface mount.
The same thing that is done to change a P-bass to a PJ-bass, plus re-routing the P-position to accommodate a J-neck pickup. Why don't you just turn it into a PJ, as that is more conventional, gets the broad tonal spectrum of a J (the whole reason studio musicians in Los Angeles had J-bridge pickups added starting a couple of generations ago), then get a more growly P-pickup, like a DiMarzio Model P
thinking this over...i don't recall ever having seen a 'face-mounted' pickup for bass guitar. i do recall lace sensor marketing a dobro surface-mounted pickup as applicable to acoustic bass guitars. Dobro Neck Sensor - Round Neck - 3/8 where would one find a surface-mount bass guitar pickup? edit: well, raise my rent: Lace Ultra Slim Acoustic Bass