You can't do it, thats why you didn't find it...
also the ability to use nested $submenu's...
which can easely be done if you just let $submenu also return the .... in front of the menu options... that way you can easely make $submenu go several menu's deep...
May i read from the help file?
Note: You can't use this to create nested submenus, it will only build one single submenu.