I've been taking the base template FBX characters from Character Creator to Blender and back using the steps in this body suit modeling tutorial:
https://youtu.be/XIxpaFUYV8Q

The only change is I put high heels on the avatar in CC, then delete the heels in Blender. This gives me the up on the toes pose to be able to make new boots via copying the body mesh. (as a side note, I haven't been able to move the pose of the base fbx character in Blender, but I'm not worried about that if I can get this question answered) 

The problem: If I just use copied body mesh and extrusion, the workflow round trip works just fine. That's fine, it's a solution I can work with if needed.

However, it would be quicker and more flexible to build things using base mesh objects (like a plane or a sphere). If I do add a new mesh object (a plane, for example) and make that into boots (or whatever), it crashes coming back into CC. This happens even if I join body mesh copied faces with the blender plane (by creating a new face between them). In other words, I can copy body mesh, extrude it, delete the original faces I copied, still works fine. If I Add a mesh object from Blender, it doesn't work. Either by themselves or joined to copied mesh. Even if I Separate the mesh to its own thing.

I'm guessing this is a rigging issue or somesuch, does anyone know? 

Or is it a bug that I'll only be able to work off copied character mesh?