Sorry for the late response, I actually found this solution about a week ago but I wasn’t able to post the answer.
You can add the link ‘rel’ and ‘alt’ options through the default menu editor by going to ‘screen options’ when you are on the ‘menus’ page in your admin area.
Tick the options ‘Link Relationship (XFN)’ and ‘Description’, this then adds those options for each menu item.
