The easyest way is to make two parts in FreeCAD and rejoin them in SH3D. This is the way I alway go when modeling with FreeCAD for use in SH3D.

This works. It means you don't see a solution besides this workaround, right? What other tool do you use for modeling?

I have not found another solution until now. But I do not search anymore, because this solution gives also some great options: you can hide easily some parts of your construction; you can easily redesign parts of your construction...