Those poor, poor Piñata. Some paper arse is going to be kicked...
As far as incompatible franchise mashups go, Viva Piñata Vs Killer Instinct must be eligible for some kind of award. Only Manhunt Vs Animal Crossing could compete, really.
Nevertheless, that is what Rare are offering via the medium of Piñata Vision cards. By downloading and printing out this set of Killer Instinct themed Vision cards and then laboriously scanning them in to the game via a Vision camera, you can get some, er, KI Piñata.
PIÑATA VISION CARDS [Rare]