Don't do it. The sub sucks as bad as the unit....and all a good sub will do is overwhelm the sucko little speakers. First thing I would try is REAL main speakers. Due to depth issues, you may need some speaker spacers for this. Amazon.com : Metra 82-4300 1-Inch Universal Speaker Adapter Brackets : Component Vehicle Speakers : Car Electronics
You can stack 'em if you need more. You can also get them made out of mdf which can be painted or finished as you see fit. Car Speaker Spacers & Rings at Sonic Electronix
I think you would see a huge difference in sound quality with a pair of Polk or Infinity 6-6.5" coax speakers. The Polks are shallow with 1.6" mounting depth so a spacer would only require slightly more than 1/2 inch depth or another space. Sound GREAT for the money. Polk Audio db651 6.5" dB Series Marine Grade Car Speakers
Now...once you have your main sound reasonably decent...then you may decide to add a subwoofer. You definitely NEED a powered sub since the Genesis power is insufficient to produce solid bass without distortion. There are a number of small powered subs on the market that are decent...I link to this one just to give you an idea of what I'm talking about
Kenwood KSC-SW11 Low-Profile Amplified Subwoofer + Enclosure 150W
The Genesis is not your BIG problem...the installed speakers are. Replace them with real speakers before you look to add crappy bass to crappy midrange & treble!