Adding another item to the nav bar when user logs in

I am designing a site which will display these five items within the navigation bar thoughout the site. Something like this - http://imgur.com/lLqN5O7 . But when a user logs in I want to add another item to the navigation bar like so - http://imgur.com/DOjOhA4. I wasnt sure what the best way would be to do this but i think mayb if I was able to switch between Master Pages. I would have two different master pages and when the user logs in it would switch to the other master page. Im not sure how to do this so looking for help here. Or if anyone has any better suggestions? Thanks
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