Sorry for the delay getting back to this thread...
It is definetly not impossible to run subwoofers (please no subbies

) from a headunit, however its not a good practice. There are several ways to do it.
Firstly, get a new headunit with a MOSFET (metal oxide semiconducting field effect transistor) (dont that just rollllll off your tounge) internal amplifier. The higher end Pioneers have them and other brands are also starting to include them. The benefit of MOSFET amps is that they are much better at dissipating heat and dealing with higher outputs and lower resistances. You are able to bridge a 4x50w PMPO MOSFET headunit to get "about" 40w RMS into one mono channel. This is ample for a modest system with a 6" or 8" sub. The high end units also have a low-pass (approx 120HZ) filter for these bridged channels so you dont need passive crossovers.
Secondly, you can use passive crossovers to split the signal between the front mid-tweeters and, say, 2x6" mid-bass (jaycar 6" woofers are perfect) in a nice efficient ported box in the boot. I have a temporary setup at the moment until I get my JL Audio 10". I am using a spare 6" carbon fiber Jaycar woofer (which used to be my mid-bass) running off a bridged 4x30w amp. The sub is getting at most about 80w, yet it goes very hard for such a tiny speaker.
One problem you will run into with this setup is that the woofers will be playing stereo, something you dont want especially if they are sharing an enclosure.
You could get really tricky and mix the two, and at the same time aleviate the probel above by bridging front and rear lefts and front and rear rights of a MOSFET amp, running front speakers off these and then combining both bridged left and right into 2 x 4ohm speakers in series. But by this stage you are better off just getting your self an amp.
If you dont get a MOSFET amp, I would probably recomend that you do try to save up for a small bridgable amp and run 1 sub, however if you really do not want the hassle, you have to make yourself up some 100hz crossovers. However to get any real effective use out of them, you will need to make up 12db/oct. onces which have a capacitor and coil for each of the 4 speakers that you will be crossing over. You have to also put a high pass crossover on the alternative speaker to the sub.
This means you will have to buy 4 capacitors, (about $12 each) and 4 inductor coils, (about $14 each). Then the trouble to wire them up, and its really starting not to be worth it. Add to this the cost of 2 woofers, and you are better off buying a modest sized mp and a single 8" sub and you will have a better result.
Hope this helps.