Joe has kindly provided the sample link required to launch the F1 asset. It should be fairly simple to inject the link into the navigation menu with a little bit of JS.
var $menu_ul = $(".fnav-menu");
var $item = '<li role="menuitem"><a href="#" data-reveal-id="myModal">Custom Link</a><li>';
Pay close attention to the middle line, the $item variable, that is where you can change the ‘Custom Link’ text to be whatever you need it to be. The ‘myModal’ part is going to be significant and will need to tie in to your Foundation lightbox attributes/properties.
This example injects the new item directly onto the first level of the menu; hopefully that is what you are looking for. Theoretically it could be injected into any of the child levels that the navigation stack supports, but a different snippet would be needed for that.
The console on your site is extremely busy…lots of messages/errors/warnings. I’d definitely look at trying to clean those up where possible.