Adding parent custom post type menu option

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 );