For your sitemap.xml file - use
GSiteCrawlerYou could create the site map before moving the sub menus out of the Nav menu to ensure GSiteCrawler can index the content. - once that content has been added it will stay within the sitemap file unless you delete it. - But I would follow the method below + then use GSiteCrawler.
You could also use the HOUSEMENU to display a submenu of a page, this is how I would set it up
Main Page
- - New Page (set to do not display in menu) eg. tab id=103
- - Old Sub Menu (set to display in menu)
Set Housemenu scope=103 in its settings
This will display the submenu pages of TabID 103 and the New Page will not display in the main menu.
Thanks,