Design Study for 2D simplified part

Does anyone know how to do a design study on a 2D simplified part? I'm required to assign material variables to the parts. And this only works for the 3D objects in the Design Tree.

However, I can not reach any of the material assignment for the parts in a 2D simplification study which do not belong to the Design Tree.

Is there any way to address that?

SolidworksSimulation