The Floor Plan Symbol of the Stair is composed of multiple GDL Objects. The Boundary and the Tread Lines is generated by the GDL Object set for the "Grid and Structure" component under "Floor Plan Display". By default this is the "Stair Grid 22" GDL Object.
You would have 2 options:
1. If you are really good at GDL, you could take the "Stair Grid 22" GDL Object, save it under a new name and modify its 2D Script to display exactly what you want. This is a potentially cumbersome route.
2. If you wanted a fast and dirty solution, you could just draw the desired 2D using Lines and Fills.
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