How to automatically create subitems based on node entries?
Try the Content Menu module. 

This module provides an easy way to add existing nodes (content) as a sub item  to menu.


  Content menu module improves Drupal's default menu management interface for tighter integration of content authoring and menu management, so content can be created and edit right from the menu interface (e.g. creating content for a menu item that has been just created.
  See the screenshot