Welcome to ETO, Azalin!
It is most likely that the "console" is designed around a μcontroller that is sending the appropriate digital signals that are controlling the PWM portion of the power supply for the motor.
As a result, there is no easy or practical way to replace the console control(s) for the motor with a simple potentiometer. That's not to say it can't be done, but we have no idea of the level of your technical skill/ability. And without a schematic of ALL the electronics for the treadmill, I think it's safe to assume the task would be just about impossible...
What is the component (or whatever) that you have highlighted with the red square?