The answer to your problem is not that simple.
You could have a servo connected to a pot or go for the electrical approach.
I got as far as designing the block diagram for an electronic load resistor by using an analogue multiplier configured as a divider connected to a MOSFET. The divider monitors the voltage and current across the divider and adjusts the gate voltage to keep the resistance constant, i.e R=V/I.
What value resistor do you require?
What does the potentiometer control?
It's probably easier to integrate the control directly into the maching rather than using an electronic pot.
You can buy digital pots but I don't think they would be any use here as you want it to be voltage controlled.