You mean to Fluther? You just type it in – you don’t have to only choose pre-existing topics. You do, however, have to ask a question that would have contemporary art as a topic, or else we would have a topic with no questions!
Well, as far as I know right now it is set up so that you can only add existing topics to your profile. Did you try doing a search for “contemporary art” in the search bar at the top of your Fluther page?