If your talking about using your Bose Subwoofer from a Entertainment center, DONT....
It is not rated enough for car audio, you will blow the cone, and potentially fry your amp.
If you "had" another speaker you wanted to add to your amp, you would need a muilti channel amp. Say a 5 channel for example. 1 channel for each door speaker, and one dedicated channel for subwoofer.
Or run like I do, 1 mono block for subs, and 4 channel for door speakers