Use the param 'show_in_menu'
in the function register_post_type()
to add this CPT to the menu. Also it is possible to use as submenu in a existing or created menu item.
Now a example to add this as submenu in a existing Custom Post Type.
// This custom post type will be added as a submenu to the 'Portfolio' menu
$args = array(
'labels' => array(
'all_items' => 'Locations',
'menu_name' => 'Locations',
'singular_name' => 'Location',
'edit_item' => 'Edit Location',
'new_item' => 'New Location',
'view_item' => 'View Location',
'items_archive' => 'Location Archive',
'search_items' => 'Search Locations',
'not_found' => 'No locations found.',
'not_found_in_trash' => 'No locations found in trash.'
),
'supports' => array( 'title', 'editor', 'revisions' ),
'show_in_menu' => 'edit.php?post_type=portfolio',
'public' => TRUE
);
register_post_type( 'location', $args );