If you have a look at this link, viewtopic.php?f=46&t=4778 I posted up info on the Sprint 400 controller In the 1st dload in the list there is a diagram which indicates you can set up a manual control via a 10k pot with the resistor across pins 1 & 2 and the wiper on 3. I don't know if this is the same for yours. I do remember warnings in other posts about not grounded to other devices and that you must isolate things (optoisolators etc) from other control circuits/supplies (they are stated in the 400 manual too) as the controller is at high voltage in ref to ground.
So that's how it works, you would just need to get hold of a board that will let you do that.
Pete