I'm not sure if you can in StarterSTS, but if you note on the project it says:-
IMPORTANT StarterSTS has been discontinued. The sources will stay up here for educational purposes. Please use my new project IdentityServer (http://identityserver.codeplex.com) from now on.
IdentityServer does support additional claims. You add the claims using the user profile - it's very easy to do so I would recommend you upgrade to this if possible.
I can't see the source for starterSTS so I'm not sure if it supports this feature. You can find Identity Server V2 here