Unfortunately due to the fact a lot of the game's events, plots and themes revolve around the AU characters regaining lost memories, abilities and items from their previous incarnation - as mentioned on the app page, all AUs do require to be incarnations of their OU selves and have a canon history of some kind. Otherwise they would be left out of a lot of game mechanics and events!

Therefore, if you don't have an canon background to pull memories and what not from - the character is not appable to the game. You're welcome to make an OC that's inspired by Hat Kid, but they too would need to have an established previous life.

Hope this clears things up!