I asked my Rav about this recently.
Most cast iron pans and griddles are sold already pre-seasoned, and there is no hechsher on them 99% of the time. Therefore, even a new pan/griddle should be kashered. He said that to do it, it should just be placed into your oven when you’re running a self-clean cycle. Then you can tovel it, and re-season the pan/griddle however you like.