Assuming it is a voltage divider referenced to output or to Vref, it should be a simple matter of setting it to a mid-range value (this depends on which type was used, and whether there are other resistors around it setting the range), and tying a resistor from the wiper or error amp input, to a DAC's output, which is programmed by Arduino. The DAC can be a filtered PWM channel.
Again, this requires knowing what the circuit is. If it's completely different, this may not be possible.
Tim