Mike,

You can draw the window frame using walls. Then you can use the attached triangle, defined with transparent material.

If you open the OBJ file with a text editor, you will find a line that says usemtl glasstranparent. Replace glasstranparent with any name from this list to change the material properties of that triangle. The list contains several more transparent material names, so you can match the glass colour with your other windows.

Hans