I've never cooked with sweet potatoes, and I've certainly never made marshmallows from scratch. I'd say you're gonna have your hands full just making that casserole.
Try Foodtv.com for some recipes. I'd assume you could bake or boil the taters and then mash em and add the "stuff" that goes in the casserole. I don't have a clue what goes in marshmallows except sugar and maybe corn syrup??