Some people do, there is some recipe where they add 1 cup evaporated milk and 1 cup buttermilk to a gallon of whole milk (they'd pour off enough milk from the gallon to add both, shake to mix). This website explains it a bit, that it is used for kids not thriving on whole milk. When I had bottle kids, I used strait up whole cows milk and they did excellent on it. If you do feel you want to add anything, just be sure to make any change slowly.
