The helpers you’ll need are wp_create_nav_menu() and wp_update_nav_menu_item().
This will allow you to create a menu and then, using a query posts loop, programmatically add every page in your site dynamically.
You can use the get_pages() helper to retrieve all pages from the DB.
<?php
// These are default args and don't need to be set explicitly.
// I've included them here for reference.
$args = array(
'hierarchical' => 1,
'post_type' => 'page',
'post_status' => 'publish'
);
$pages = get_pages($args);
// loop goes here
// foreach or while will work, whatever you are comfortable with
?>