Is it possible to add a menu to a menu? I've created a "Staff menu" with various menu items. However I want to add it to the "Main menu" as a drop-down list (e.g. set Staff as Vertical, while Main is the navbar / horizontal menu).

Is it possible to just add a menu to a menu by creating a new menu item in the "Main menu" and linking it to "Staff menu" ?
You can use this module http://drupal.org/project/om_maximenu then create parent menus and its child menus .Even you can create blocks inside with the sub menus.