There is no such thing as "overclocking a fan". The fastest rate that they will spin at will be achieved by plugging them directly into a 12v output from the power supply (although you may need an adapter for the cable).
Most fan profiles administered by software will start a fan running at a partial speed (lower voltage) to lower the noise floor, and then speed it up (raise the voltage) as temperatures increase.
To run a fan at full throttle, the software will increase the voltage to 12v.
you can achieve this manually by connecting the fan directly to the PSU.