Wordpress - Is It Possible To Add Custom Post Type Menu As Another Custom Post Type Sub Menu

Yes. When you register your post type you need to set show_in_menu to the page you would like it displayed on.

Adding a custom post type as a sub-menu of Posts

Here we set the "movies" post type to be included in the sub-menu under Posts.

register_post_type( 'movies',
    array(
            'labels' => array(
                    'name' => __( 'Movies' ),
                    'singular_name' => __( 'Movie' )
            ),
    'public' => true,
    'has_archive' => true,
    'show_in_menu' => 'edit.php'
    )
);

If you have a taxonomy registered to the custom post type it will need to be added to the page as well.

In add_submenu_page() the first argument is the page to assign it to and the last is the menu slug.

add_action('admin_menu', 'my_admin_menu'); 
function my_admin_menu() { 
    add_submenu_page('edit.php', 'Genre', 'Genre', 'manage_options', 'edit-tags.php?taxonomy=genre'); 
}  

Adding a custom post type as a sub-menu of another custom post type

To add the pages to another custom post type include the post type's query string parameter along with the page names.

To add the CPT Movies and its taxonomy Genre under the post type Entertainment adjust the code like this.

edit.php becomes edit.php?post_type=entertainment

edit-tags.php becomes edit-tags.php?taxonomy=genre&post_type=entertainment

register_post_type( 'movies',
    array(
            'labels' => array(
                    'name' => __( 'Movies' ),
                    'singular_name' => __( 'Movie' )
            ),
    'public' => true,
    'has_archive' => true,
    'show_in_menu' => 'edit.php?post_type=entertainment'
    )
);

add_action('admin_menu', 'my_admin_menu'); 
function my_admin_menu() { 
    add_submenu_page('edit.php?post_type=entertainment', 'Genre', 'Genre', 'manage_options', 'edit-tags.php?taxonomy=genre&post_type=entertainment'); 
}

Our custom post type:

$args['show_in_menu'] = false;
register_post_type('custom_plugin_post_type', $args);

Add him for existing Custom Post Type ("product" for example):

$existing_CPT_menu = 'edit.php?post_type=product';
$link_our_new_CPT = 'edit.php?post_type=custom_plugin_post_type';
add_submenu_page($existign_CPT_menu, 'SubmenuTitle', 'SubmenuTitle', 'manage_options', $link_our_new_CPT);

Or add for our custom plugin menu:

// Create plugin menu
add_menu_page('MyPlugin', 'MyPlugin', 'manage_options', 'myPluginSlug', 'callback_render_plugin_menu');

// Create submenu with href to view custom_plugin_post_type
$link_our_new_CPT = 'edit.php?post_type=custom_plugin_post_type';
add_submenu_page('myPluginSlug', 'SubmenuTitle', 'SubmenuTitle', 'manage_options', $link_our_new_CPT);