You can, but most of those poor quality treadmills are meant for light use (i.e. running at high RPM for 30-60 mins per day, 3-5 days a week). If you try to run the same treadmill at low RPM for several hours per day every day, you'll burn out the motor pretty quickly.
That's not to say that people don't try though, there are plenty of DIY treadmill desks made out of exactly what you describe.
That's not to say that people don't try though, there are plenty of DIY treadmill desks made out of exactly what you describe.