Yes you could use a potentiometer to control the voltage the fan gets (aka the speed). Just make sure you get a potentiometer that can handle 12v at least and whatever the amperage rating the fans can pull since I believe the 12v lines can pull up to 5amps, someone correct me if I'm wrong.
you can find potentiometers online at places liek mouser, sparkfun, digikey and the sort.