Integrating a sub

Hey, I'm not sure if this is the right place, or even community to post this, but since it pertains to my DJing/mixing setup I figured I might as well give it a shot.

I'm running my tunes from my macbook thru an M-Audio FastTrack Pro. I currently have 2 Mackie MR5 [MK2] 5.25 monitors and I'm looking to add a sub because I love my bass.

I currently run the Mackies through a pair of 1/4 cables into the 1/2 of my FastTrack. Before I go spend some change on a sub, I was wondering if I would be able to run it through the RCA 1/2 on the FTP and still have everything work.

I'm running Traktor Scratch Pro and I have my main outs as 1/2 and my headphones routed to 3/4, even though they are plugged into the front of the unit.

Sorry if this is confusing, or the wrong community completely. I'm pretty dumb with this kind of stuff, and I don't want to spend money on something to find out it won't work with my setup.

Thanks!