To make sweet potato filled pierogi, I just used my regular recipe (sort of) for Potato and Cheese Filling and substituted sweet potatoes for the baking potatoes (Russets). First, boil the sweet potatoes in their skins and then rice (or mash) and add to the rest of the ingredients. You may want to use a mixture of both potato types, because the sweet potatoes have a lot more moisture in them, making it more difficult to pinch and seal. Feel free to experiment with quantities. I made a small batch with just a couple of small sweet potatoes. added a bit of cheese and onions without measuring. They were sweetly delicious!
Sweet potato pierogi served with sour cream and Pennsylvania Dutch style red cabbage 
Frying the sweet potato pierogi in butter with onions