If you mean you want to do part whole milk and part formula (formula mixed in water first) yes, it's fine.
If you mean to mix formula with the milk... 4 oz milk and a scoop of formula.... well, you could, but it would be a huge amount of calories. They do something similar to this with breastmilk with NICU babies, it's called fortifying. I wouldn't do this without a doctors orders... so no.
at 1:53 PM on Jan. 5, 2010