Sorry, no-can-do. The customized menu commands that appear in the context menu are global. They must be done at design time (well, actually manually outside the designer), and are not alterable in the editor. We have wanted to be able to do this multiple times.
Of course, someone might always find a way to do it (just none I'm aware of), and if so, we'd appreciate it if you'd post here to let us know how it is done!!