Is there a function, action or filter that I can use to add a third level drop-down menu to the WordPress admin menu.
For instance, right now in the sidebar menu, there is a menu for posts and under posts there are sub-menus for editing posts, adding a new post, categories, and tags. There is something similar for Pages.
What I would like to do is to add a menu item called Content and place underneath content Posts, Pages and my Custom Content Types and underneath each of those entries the relevant sub-menus (editing, adding, etc.).
I would like to do this through a custom plugin that I create. The problem is, I can't find any information on how to add a third-level sub-menu.