Need some guidance on vfd control for mill while also using existing control. Potentiometer?

My mill has 4 motors between the power feed and the horizontal attachment. I really only want the main spindle on a vfd and I want to be able to control the speed with the vfd wired to a switch, or something that looks like a potentiometer, that I will run to the existing control box. I would still like the power feeds to be controlled by the control box as well as start and stop from the motor.

Is all this even possible? Because I'm guessing that power needs to be ran to both the vfd/motor...

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