Basically you would use the pin that switches the buzzer to be at the base of a transistor. All that will do is "tickle" the transistor and switch it off and on. That means it will switch the current in the other 2 legs of the transistor off an on: that will be the voltage supply to the motor.
So when the buzzer would make a noise now, instaed the transistor will switch the supply the motor on.