If I understand your question correctly... you can't.

Or, at least not with stock SH3D models. Instead, you'd need custom 'half' doors and windows, to give the cut-through look. Set your wall height to 4 feet (122 cm), place the special door and window objects, and you're done.

I don't *think* such models exist here or in the Sourceforge database; please correct me if I'm wrong. If I get a chance, I may try my hand at these later today.

maz