If you go to File\GDL Objects\Open Object by Subtype, you get the Dialog in the attached image.
Here you can find the Window-Door Markers, and you can open them for editing, Save as another name to any place you want, and you can modify that.
If you save it into a loaded library you will have in the list of selectable Window-Door Markers, so you don't have to search for it with the Load other Window-Door Markers command.
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