Oracle Forms ships with a ready-made menu module (menudef.mmb) that has the exact same menu items as the default form menu. Since the menudef.mmb menu module is a file copy of the default form menu, you can modify it as much as you like. You need not worry about permanently changing the default form menu itself; since it is an internal component, and not a file, it will alway be available to you in its original state.
If you modify the menudef.mmb menu module, note that its item commands are implemented with DO_KEY Built-ins (which execute key triggers, thereby allowing end users to execute commands from either the menu or the keyboard). To maintain consistent functionality, be sure to use DO_KEY builtins when you add menu item commands to the menudef.mmb module.