You won't be able to run the 5 volt up to 7, with a simple resistor: the 5 volt line comes directly from the power supply.
7 volts is achieved in the difference between the 5 volt line, and the 12 volt line (12 - 5 = 7, see?). You might be able to mod your circuit to do this, but you would have to know what you'r doing, otherwise you might end up shorting the 5 volt line from your PSU (and you'll burn something in your controller).
Why don't you just use the 7 volt trick directly on that fan? It won't be adjustable, but it'll work. Alternatively, you can make a switch yourself that does this (want a diagram?).
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