It is safe to combine milk and liquid formula
(but do not add milk to formula powder).
After a baby is 12 months of age, and is eating a variety of baby foods, he can be weaned off formula and start whole cow milk. The weaning can be gradual over a week, in which more and more cow milk is mixed into the formula each day. By the end of the week, the baby's drink will be mostly or all cow milk.