If you have regular single coils now, and no extra wiring for tapping the coils, then each pickup has only two wires - one to the switch and one to ground (usually at the back of the volume pot. If you now put humbuckers in (I guess you were referring to single coil sized humbuckers, otherwise you would have to route the guitar) follow the manufacturer's wiring code, use the series wiring guide and put one wire to the switch were the old pickup was and the other to ground (for Bare Knuckles that would be red to hot and black to ground, white and green soldered together and taped - just taking an example here as BKP do not make single coil sized humbuckers).
Of course, if you want to split the humbuckers in certain switch settings it gets more complex.
Cheers Stephan
