Ron's first idea is the only decent way to work out the problem, using the motion detector output to jumper around the circuit. However, no matter how you use the motion detector in this circuit, you'll have to have it permanently connected to the 120v and have the output drive a relay that's connected between the source and the load. Reason? The motion dectector derives its power from the source and if you're always using the other switches to control the light, you'll interrupt power to the motion detector, either rendering it inoperative or in the case of some models, resetting them to some default mode of operation (like, ON all the time).
Even using relays and constantly powering the motion detector, you CANNOT just substitute the relay output for one of the switches in the lighting system. For the same motion detector that turns on the extinguished light when activated will also turn off the "burning" light when activated!
Dean