I'm not sure about custom titles; I always thought I was "A Friggin' Genius" because I'm, well, a friggin' genius.
Per board policy, users do not have the ability to change their own status. It's an esthetic thing - nothing seems weirder to me than a guy with 100 posts calling himself "FOOTBALL GURU!!!" or some such nonsense. I realize that most of you would be reserved in your self-nicknaming, this policy is for the kind of jerk who would call himself "A Friggin' Genius".