Not completely advisable. But if you REALLY want to do it, you probably would be best using an external power supply. Pick up a cheap Radio Shack 12V DC power supply, cut the connector off and add the appropriate connector(also from Rat Shack). You will then have to figure out how to mount the fan inside, and make a hole just big enough to get the wire from the external PS though.
The next consideration is the static pressure inside the case. Chances are you will want to have the fan as exhaust, but if it is a 5200RPM fan it will likely be overkill. You may want to consider adding a Rheostat(sp?) to control the speed.
