I suppose you could, but I think it would take sphagnum quite a long toime before it broke down to the same level as the product you find marketed as "sphagnum peat moss"
A better question might be, "Why would you want to?" Peat moss is something like $7.99 for 2 1/2 cubic feet at garden centers. 2 1/2 cubic feet will last you years. If it's for fish use, just make sure it's organic, or specifically excludes any added fertilizers, water retention gels, or anything else. I've used garden center peat in my tanks for years, and I have no reason to think it's inferior to aquarium store peat in any way.