You can apply different Materials to separate bodies in a multi-body Part by expanding the cut list folder to the desired body and right-clicking as shown below.
However, I'm not sure that answers your question. If you're trying to apply a custom property to a body you can do that in the cut list properties. Does this help?
This helps on a separate issue Glenn, thx. I think I just ran into a glitch in that I had to
re-click on the density in a custom material, then hit apply. The weights changed to correct values
once I hit apply again. Maybe the first time around I did not have the new density value applied correctly.
You helped me in that I have parts that have different extrusions, so different materials are needed.