Currently, css.frame.PopupMenuControllers are not supported for *root*
PopupMenus (== a PopupMenu that is activated when a MenuBar item is selected).

There may be several scenarios where an extensions developer wants to fill his
own root menu at runtime (and this will be more easy and give him more
customization than just adding a new menu in the Addons.xcu and then
disabling/removing items at runtime).

There is an API way of achieving something similar: The css.awt.MenuBar is
accessible as a property of the UIElement (not null when it is a MenuBar, of
course) returned by XLayoutManager.getElement().

But this way implies accessing OOo menu bar, what in general is dangerous; and
it is much more work than a PopupMenuController: one has to access the MenuBar
via the LayoutManager, create a new MenuBar item, and add the root PopupMenu to
the MenuBar.
This PopupMenu is transient, as the LayoutManager destroys and
recreates the MenuBar under certain circumstances, so one has to listen
to frame actions and recreate the MenuBar item added...

In conclusion, a css.frame.PopupMenuController is easier and safer.
This feature should be available for both add-on root PopupMenu and Office
MenuBar merging.