RE: Adding control on the top of existing NameMapping tree
There are two options in your case:
1. Map your panel object manually from the Object Browser. When in the Name Mapping editor, drag children of the window where this panel is located and drop them onto the panel.
2. Use Extended Find for objects which should be the panel's children. In this case, you don't have to map the panel at all (see the "Using Extended Search Criteria for Mapped Object Identification" help topic).