Maybe this can help.
You have two options, as I see it.
First, just buy an amp and use it to drive your current speakers. It's just as Pete says except the you run the speaker wire to back to the front to connect to the door speakers. You'd just splice them into the stock speaker wires for easiest install.
Slightly less work is a sub install, as far as wiring goes. You don't have to run speaker wire all the way back to the front, just a short run to the sub.
Now, you'll get a little more bass than now going the first route, but a 6x8" speaker is not capable of creating a lot of bass, no matter how good. Thus, Dave's (dnewma04) recomendation for a dedicated subwoofer. Depending on how much trunk space you ware willing to give up, you can get a sub and use the amp for it only, and use the head unit for your door speakers. That'd be a relatively easy system to wire (either for yourself or = less money at the audio shop).