There is not an a simple way to boost the voltage of the aux output, no.
The aux output can do a max of 5 volts, but it also does less than that if you are varying the speed of your fan, so it is not a fixed voltage. The way it modifies the voltage is through high speed switching, which I suspect your voltage booster does not like. Secondly, even if your booster worked, that extra voltage has to come from somewhere - and it comes through an increase in current. You could find yourself exceeding the limits of the Aux mosfet, or of the TCB regulator.
It's not impossible to boost the voltage, but you'd need a 7.4 volt source, and then feed the Aux output into another mosfet circuit, which would use the Aux as a signal only, to control the higher voltage. But that requires you to make another circuit.
If you don't need variable speed control, just on/off, then you could use one of the General Purpose I/O ports to control a relay, which would then switch your 7.4 volt source.