Is there a way to display a custom post type menu item as “users” submenu?

Solution 1 (not a good idea):

 add_users_page(
   _x( $this->page_title, "page_title", $this->text_domain ),
   _x( $this->menu_title, "menu_title", $this->text_domain ),
   $this->capability,
   "edit.php?post_type=my-post-type"
);

Notice : This code is used in a class.

When you are on the page, it can have some bugs with the menu, it´s a good idea for specific page

Solution 2 :

register_post_type(
  'my-post-type',
  array( 
      'show_in_menu' => "users.php"
  )
);