I do not have D&W Builder but I know that you can model a Door Handle and save it as a Custom Door Handle type of Custom Component using File\Libraries & Objects\Save Custom Component menu command.
Maybe if you do that the Custom Component will appear in the list of selectable Custom Door Handles in the D/W Builder Settings Dialog.
Edit: The type of Custom Components you can save this way will depend on the loaded libraries.
For example, on Windows, the C:\Program Files\Graphisoft\ArchiCAD 13 INT\ArchiCAD Library 13\Custom Object Macros 13 is the place that contains the subtypes defining the types of Custom Components that can be saved. This has to be loaded for them to appear as options - of course they do if you just simply load the ArchiCAD 13 Library.
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