The outdoor sub has line level (RCA) or speaker level inputs or both? Line level would probably be better signal wise, but since you already have the wires for the other speakers run, you could tap into them to get signal for the sub. This will not affect the impedance of the line or the amount of power drawn as long as the sub is powered. If it has speaker level inputs, you might be able to run the wires to the sub and then from the sub to the speakers, though I would check to see what the sub's high pass crossover is first.