You can still use standard Gtk menus with sub-menus, it is just not recommended. Applications designed for the new UI guidelines are very slick, but you can still use all traditional Gtk elements to make ports easier. If something behaves in a totally incompatible way, then it has been implemented as a new Hildon widget instead of changing Gtk functionality. Dialogs are probably the biggest case of changed functionality, but they should generally still work ok without changes.