In addition to the method of adding new shapes to the diagram we have now from the palette, it would be good to have a menu to do the same thing as appropriate on different objects.

For example, if you right-click on a Composite you would see an "Add" menu that would show "Service," "Component," and "Reference." If you right-click on a component, you could add a Component Service, Component Reference, etc.

This would also work well towards making the editor more Section 508 compliant for users who can't use a mouse and can only use the keyboard or assistance technologies.