Dried (powdered) Milk in Fluid Milk Processing
I haven't heard of adding powdered milk to fluid milk as part of it's processing for you're typcial gallon of milk at the store. Because the milk from every animal varies a little, part of the processing of raw milk into what you get in the sore is called standardizing. Typically milk is standardized to a specific fat and solids-non-fat (proteins, sugars) based on the product being made. Whole milk where I live, for example, is standardized to have 3.5% milk fat - or at least it was when I took dairy processing in college. It could be this is what you are refering to.
With other products, adding powdered milk can be used to change the protein content or the viscosity. Our you refering to a product for the baking industry?