How can I create a DC-DC voltage multiplier
You could use an MC34063 boost converter circuit to do this cheaply. 



The parts values will be different, but the circuit will be the same. Calculate the parts values using the equations in the datasheet page 11. If the port can supply 500mA without complaint, then you could get about 100mA @ 12V into the fan assuming 50% efficiency. USB only guarantees 100mA without negotiation.