yes, but why would you ?

wire in a potentiometer?? to get more of a manual control of the spindle, obv you can tweek the spindle speed via mach but, to have the ability to fine tune the spindle manually via a dial seems a logical option,  but as yet I don't know if I want that in my setup. I'm just going through some online manuals regarding the wiring up of inverter drives, am looking at the parker ssd650, as the stepper drivers are parker thought might as well keep to the same make, this is the manual -  http://www.inverterdrive.com/file/Parker-SSD-650-Easy-Start-Guide, section 10 shows how to wire up a potentiometer and reading between the lines how to wire mach into this drive, I'm thinking that using mach as the potentiometer then my 0v to input 1 and speed ref signal (10v) to input 2 (AIN1), the spindle will only go in cw direction but that's ok.

you can command any speed you like with mach and fine tune via the screen. But yes you can use a pot.