So you want to disconnect the rear door speakers and add speakers to the shelf and use the exisiting door speaker connectors?
Don't ruin a shelf by cutting into it house speakers. Your best bet is to replace the fronts for better quality and leave the rear doors.
If you still want to put speakers on the shelf then either cut the connectors off the rear door speakers and attached a new length of wiring to the rear. Or you can buy block connectors which attach to the existing connector and attach a length of wiring to the rear.