Sweet potatoes can be frozen whole, in slices or mashed. The National Center for Home Food Preservation has simple tips on how to do so.
To freeze sweet potatoes, choose medium to large ones that have been cured for at least a week. Sort according to size, and wash thoroughly. Cook until almost tender in water, in steam, in a pressure cooker or in an oven. Let cool, and peel the sweet potatoes, cut in halves, slice or mash. To prevent darkening, dip whole or sliced sweet potatoes for five seconds in a solution of 1/2 cup lemon juice to 1 quart water. For mashed sweet potatoes, mix 2 tablespoons orange or lemon juice to each quart mashed sweet potatoes. Pack into containers, leaving 1/2 inch of space between the potatoes and the lid. Seal and freeze.
To freeze baked sweet potatoes, wash, trim, and bake them in an oven at 350 degrees Fahrenheit without peeling until slightly soft. Let cool, remove the peel, and wrap individually in aluminum foil. Place in polyethylene bags, and freeze. To eat, remove from freezer, and finish baking in an oven at 350 degrees Fahrenheit immediately before serving. Leave the potatoes wrapped in aluminum foil when baking.