what you need to do is the following
create database table to save all your menu items , once the user want to add the new page you need to let him choose the parent page ,
now write some code to populate menu items from the database.
i have article here you need little modification and it will match your requirements
http://engthunder.wordpress.com/2011/10/17/dynamic-menu-from-database-and-more/[
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