You can, via the API, log someone in with only a username. In Kentico 7 it was:
CMSContext.AuthenticateUser(TheUserName, true);
You'll have to look at what it was for whatever version you are using. The rest will be up to you, just make sure you make it secure and hidden as you can theoretically log in as the admin if you trick it to have "TheUserName" be the administrator account.