You can finish cut 3D in expanded PVC with a regular ball end mill - just like you would machine 3D in any other substrate. It is going to be a bit sandy/grainy no matter what tool you select.

Just like I tell anyone cutting a new material or strategy - cut a sample! Try it out and observe what is going on. Mess with move speeds and RPM until you are happy with the results. In this case, it shouldn't be too difficult to find the sweet spot.

-B