Rig a BJT to drive a fan
Something like this might work for you: 







simulate this circuit &ndash; Schematic created using CircuitLab

R1 is not strictly necessary but it reduces the chances of you frying something. 

Q1 acts as an emitter follower to buffer the pot voltage. Q1 may need a heat sink if the fan draws a lot of current. R2 enforces a minimum voltage setting- since most fans won't start if the voltage is too low. Adjust as required (probably upwards). The circuit as shown gives about 2V to 11V+ at the fan.