Can't do that with pro. You could fake it in as a bunch of segements like a chain. Put a datum curve around the pulleys then using Mechanism make slot connections between the segments and the curve. This will work but will be lengthy to set up.If you make small thin segments and pin them at the adjacent edges it will look pretty good.
In reality just make a static belt and move everything else. The Kinematic soultion doesn't depend on the belt anyway. All analysis will be correct, and very few will notice the static belt anyway.