What is a good substitute for full cream milk powder?
I wouldn't add whole butter if all you're after is some extra fat.

You could try gently rendering down some unsalted butter to get the fat you want.

Do this on very low heat, do not let it burn, take it on and off the heat periodically until it barely begins to foam, then skim off the fat with a small strainer or a tea ball.

Then, save the clarified butter for something else. 

A small quantity of that stuff goes pretty far, so you'd have to do some experimenting. A stingy teaspoon per cup of skim milk powder sounds like a reasonable starting point, but try a small batch first for testing.