You'd have to add the following line to the $main_navbar array of core/includes/templates/navbar.php (lines 26-29):
'/link' => array(1 => 'Title', 2 => 'name')
where:
/link is either the internal link to the custom page, or the URL of your external page
'Title' is the link title (which will appear in the navbar)
'name' is the page name (the $page variable on your custom page, if you don't have one just leave it as name)
I want to add a custom page at the navigation menu or a link to another website on the navigation menu. How would I go about doing this?