Ski flying does not require a rating or endorsement, but you should spend a few hours learning the tricks and traps with an instructor or experienced ski pilot before embarking on an adventure whose demands are easy to underestimate.
Most general aviation ski flying happens in small taildraggers. A tailwheel endorsement is a gateway, and the guidelines for operating on skis are similar to those for tailwheel aircraft on a soft field. Still, flying a ski-equipped airplane is a departure that requires attention.
The main differences are the lack of brakes and the snow, which affect every phase of ground handling. In addition to keeping you moving when you want to stop, and acting like an anchor when you want to move, snow makes turning a challenge.