Sphagnum moss has some of the greatest water-holding ability of any plant, plus it's acidic as hell when it's fresh and then for a while. A hide box on one end with paper towels, recycled paper bedding, or aspen is the route to go. Your snake's water dish should provide all the moisture it needs; if not, then mist your cage ocassionally.
There's no need to use Sphagnum moss in a kingsnake's cage, and this is coming from a botanist. If your breeding and need to keep the eggs moist and warm, then maybe so - I don't breed and others can address that. But as a place for your snake to hide on the cool end...I think it's a mistake.
I lost my first king years ago by providing it with a nice, moist place to hide...and that's just what she did; that and too much airflow caused an upper respiratory infection that killed her before I knew what had happened. Today I would notice it right away and get it treated, but the point is that I don't recommend using a plant such as Sphagnum in a kingsnakes cage unless it's been boiled and dried and then used simply as a cushy substrate.
Think about it.
Peace.
-John