Theoretically, yes. I've never tried this, but the way it was explained to me was . . .

You'll need to start by using a 6v power source. Try to get your hands on an old 6v battery. Hook the fan up with an ampmeter and measure the current drawn by the fan. Divide 6 by the reading you get on the meter to determine the resistor you will need. For example if the meter read .5 amps, you would need a 12 ohm resistor (6 / .5 = 12). Be sure the resistor is able to handle all of the current that the motor will draw. Then all you have to do is put this resistor in the power line going to the motor.