As far as I can tell, My retracts are made to operate on the system as a whole.  I can see no way of designated one battery to the Receiver and servos.  And Just one battery to the retracts.  It seems to do what Jaka suggests, one would haft to hack into the retracts and totally rewire solder and whatever else.  So that you could just get the signal from the receiver but not the voltage. And get the voltage from an external pack into the retracts without running it through the receiver.  If it were possible, It would void the warranty.