A project I inherited had a dozen custom post types. Trouble is, they all come off the admin menu sidebar separately. It's not very tidy. Is there a plugin where I can make these subitems of a parent menu, or is there a way, programmatically, I can edit my theme's functions.php to make it make these as submenus?
On the function to register a new custom post type can you set this CPT as Submenu to a exist menu item. Use the param
You find the string for the items on mouse over, like