There is a tutorial in another thread that shows how you can mod the pot inside the servo to add a magnet to the pot's shaft and a sensor underneath it that can sense 360 degrees of rotation. But you will also need to replace the electronics with your own, to include a microcontroller and an H-Bridge. I still find the continuous rotation servos too noisy, and for a competition robot they're too slow most times. I have replaced the CR servos in my robot with Faulhaber motors (surplus, see the Electronic Goldmine) that are faster, silent enough, have internal encoders and cost the same as a servo. But you also need to buy a hub to mount your wheels.
Now I'm still looking out to find miniature DC motors (think 1 inch square robot) that are cheap (under 10 usd / piece).