A common problem. You wanna add a link in your topbar menu but is not possible. Permalinks are not editable in full, pages are added automatically to the topbar menu, and so on. Ok, the neat solution is the Page Links to plugin. It simply adds a new option in the Pages editor with a form input field for adding any external link you wanna use instead a post or page.
Install the plugin (you have two options, go direct to the plugin author page, or search for it inside WordPress Dashboard > Plugins > Add new > Search) and all is one. The next time you edit a page or Post you will have a field with the link option.
Author page: http://txfx.net/wordpress-plugins/page-links-to/
