Replace the pot with a fixed resistor (they usually specify 5k or 10k), leaving the wiper connection free to
drive from a low-pass-filtered PWM signal.
But first check the voltages the unit is using with a real pot in position. Needs to be in 0..5V range.
Most controllers detect open or short circuit in the control pot so you have to provide a real resistor
to avoid triggering error-shutdown.