Does anyone know if a supercharger pulley upgrade is made for the Nissan Xterra that is warrantied by Nissan?

While I do not know for sure, I would highly doubt that one is made by Nissan. NISMO is Nissan's high-performance company, and would have been responsible for making the pulley you have on your Xterra currently. Upgrading the S/C pulley will increase the boost to your engine. While this will give you more power, it also places greater strain on the engine. This is not something Nissan wants happening, especially if the vehicle is under warranty.