Maybe you have already found out, but there are two standard ways of representing furniture items in SH3D: try to toggle Preferences - Furniture icons in plan. The Top view may not be perfect for correct architectural symbols, but a decent approximation which allows any imported object to be represented in the 2D view.
For models with additional SH3D properties, i.e. defined in a library file (.sh3f) - it is possible to have customised "planIcons". However, you may not be able to find find an existing library with exactly the symbols you want.
If you still really wish to create an exact representation the way you describe it, you could make your own by:
1) Creating a library (with the Furniture Library Editor).
2) Editing the contents of the .sh3f file (which is a .zip file).
- add a planIcon# property in the PluginFurnitureCatalog.properties to a model in the library,
- add a file with the plan icon you want.
There is more info on advanced editing in the forum/documentation if you wish to try. Where to start really depends whether the above sounds very easy, or very complicated to you...
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