Can you put a sub-menu in a panel launcher menu?

I was wondering if it is possible to make sub-menus in the panel
launcher plugin.  

Currently, if you add a launcher to the panel, you have the option of
making a menu that is activated when you click the up arrow next to the
panel icon.  I would like to have this option on each item in the menu,
so that there can be sub-menus.  If this is possible, I would like the
sub-menu to expand towards the right, to keep with convention.

If this is a hack that hasn't been done, what files should I edit?

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