If you wanted to do this for an FRC robot, well, you can't because only the digital sidecar may connect directly to the PWM motor controllers. I don't believe the RasPi has PWM support built in, but you could use the CPU to manually generate the PWM signal. Alternatively, you could get something like this which uses the i2c bus to control multiple PWM outputs:
http://adafruit.com/products/815
EDIT: There is one PWM output, but that may or may not be enough for your needs.