You can create masks on other layers, using the shape/mask tools, but you won't have all the animators or controls of a shape layer. Can you describe in more detail what you are trying to accomplish?
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.