I think I understand your question, so I'm going to repeat what I think you'd like to do.
Your machine (Singer 99) is electric and you'd like to put it in an empty treadle cabinet. You would like to use the treadle for the foot pedal (another word for controller).
If that is what you are asking, then yes, it's possible. Many commercial sewing machines have a foot pedal (controller) that is wide enough for both feet to fit on. My guess is that it would take a lot of experimentation to get it to control the machine speed correctly. This is kind of a weird idea, but you could wire a pedal from a Touch and Sew and put it under the treadle. When you push on the treadle, it would push down the pedal. You would have to disconnect the bar that goes from the treadle to the wheel as it's usual way of working is to turn the wheel in a continuous motion, and the usual way the treadle works is to continually move up and down.
Sure it could be done. The question is will it work well enough and with enough precision to be practical.
Someone will come up with a better idea, I'm sure.
bkay