It's not as late now so I'll elaborate. Yes, you can hook up a sub to your factory stereo. You'll need to buy one of those converters. The converter splices into your speaker wires and converts the signal sent to your speakers into RCA jacks so that you can hook up an amp to it. So as long as you can get to your rear speaker wires you should have no trouble at all. The quality might drop off a little using a converter, but since you are just running a subwoofer you shouldn't really notice it.
Scosche SLC4 Four-channel line output converter at Crutchfield.com
This one looks a little nicer than the junk ones I'm used to seeing, but that is the thing you will need. Any local stereo shop should have one, or be able to get you one, for pretty cheap.