Add nonexisting pages to navigation

Try the filter ‘page_link’, and manipulate the url if id is over $wpfbId

http://codex.wordpress.org/Plugin_API/Filter_Reference

applied to the calculated page URL by the get_page_link function.
Filter function arguments: URL, page ID. Note that there is also an
internal filter called _get_page_link that can be used to filter the
URLS of pages that are not designated as the blog’s home page (same
arguments). Note that this only applies to WordPress pages, not posts,
custom post types, or attachments.

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