To add more items to the menu that pops up on the class diagrams, edit UMLPopupMenu. This menu has been broken into a number of methods each generating a particular submenu. The ActionListener that is invoked are stored either in org.acm.seguin.uml.refactor if it is a refactoring, or org.acm.seguin.ide.common.
The reason to place it in ide.common is that we want to isolate changes that are specific to a particular release of an editor. This makes it easier to add the feature to other editors.