Right now there really isn't a way to do that. That's something we have plans to implement in the future, but the modal is part of the module only right now, and module's can't be loaded into menu items. It would take a bit of work to get the modal popup to work from a menu item.
With that said, if the Login module is being displayed on the page somewhere, you may be able to get the popup to work by just calling our Javascript that's already included on the page. You'd have to have the module on the page though for that to work (You could honestly use some CSS to hide the module though).
Let me know if that helps explain and you'd like more information on the Javascript you'd need to add. I don't know if you can do Javascript in a menu item though, so it'd be a complete test and may not work at all.
Thanks,
Alex