Internal relative links in Menu by using http:/alias/path/to/link
You should be able to go to /admin/structure/menu, select the menu you wish to add the link to, and list links. At the top select Add link and enter the title and link. You can use either the alias or system path. Then on the list, place the new menu item where you want it. It should not overwrite the existing menu entry.

Do not prefix the links with http:.