You can do it a few ways but for a Pot you will need analogue inputs to the computer. Several ways you can get that, PLCs over ModBUS, Peter Homann does a ModIO board that has analogue and also there is the PoKeys that has analogue (not sure if they are working well enough yet)
Another way to do it is have two or three momentary switches, one for Up,one for down and the other for reset, you can then make up a brain that steps up or down through the speeds when you keep the button pressed.
Lastly you can get a rotary switch and make up a Brain to set the feedrate to a different amount depending on the position of the switch.
The analogue/pot option is probably the best but the rotary switch comes a close second and is obviously a lot cheaper.
Hood