Emmanuel:

Don't worry too much. There's a very workable workaround, of course: Group the items, Copy as new furniture, copy and paste into a new instance of SH3D (I generally have two open just for that purpose), then Export to OBJ and re-import as door/window.

But just in case you have some time left... it would be nice if the extra steps weren't necessary, because it would be more flexible.

Hans