I'm unsure exactly what you meen, are you talking about putting a resister in the wiring (ohm resister) or a physical resistor to hinder the spinning of the blades?
If your talking about electrical resitance:
V=IR where:
V is (voltage, measured in.... you guessed it volts)
I is (current, measured in amps)
R is (resitance, measured in ohms)
so I = V/R
if you lower the resitance and voltage remanes constant (which it will your power supply does that for you), then your current or amprage will increase. however, your talking about adding resistance, in this case you would DECREASE amprage going to the fan.
if you want to decrease resistance in you fan the best way is to use silver wiring and as short as possible...what will this get you? nothing noticeable...you'd probably need an expensive volt meter to tell the difference
also i'm not exactly sure but I think the amp draw would be the same if you put physicl resistance on the fan, it's producing the same torque regardless if it's moving or not (that's why torque is much better the horsepower for measuring...anyway).
