Yes, a pie pumpkin is fine. With the kitties being so tiny you don't need a whole lot per feeding. An ice cube size would be ok mixed with their food. Some kitties will actually eat it on it's own but not many.
If you don't feel energetic enough to cook the pie, btw, I cook mine like I do potatoes and then mash it afterward. You cook yours in the oven? Neat! Anyway, you can find cans of just plain pumpkin near where you would the pie filling. It clearly states just pumpkin in the ingredient list.