Thanks for the wishlist item Butch! I will add it to the list, to summarize to ensure I have your wish correctly;
In the “Lites” tab of the Doors dialog box, you want the same “Custom” option that is currently available in the “Leaf” tab that allows you to import a .skp, .dwg or .dxf file that illustrates the sidelite.
As a side note, if this sidelite is always paired to the same door, you can build it as one unit in Sketchup and import it into Envisioneer using the “Custom Block” option when you “Add Element” to doors. That is another way of using the custom sidelite in Envisioneer.