To copy a shape layers path into a mask,
1. duplicate the shape layer
2. select the duplicated shape layer then Layer-Autotrace
3. go to the Autotraced layer in the top of the Comp, Copy its mask & paste it onto the shape layer you want it to work on.
4. turn off the Autotrace layer & the duplicate layer if not required.
You Can Do It! -Rob Schnieder