Products like half&half, creamer, heavy whipping cream are made exactly like that.
Raw milk from a cow will be 3.3-4% fat content depending on many factors (breed, time of year, type of feed they receive). This milk is then separated into cream at 40% fat and skim milk at 0.5% fat. From there the streams of cream and skim are remixed according to whatever product they are looking to make, putting in the lowest legal amount of cream to meet labeling requirements. The extra cream is turned into butter.
1% milk is typically 0.97%, 2% is 1.97%, and whole milk is 3.25% (slightly higher in California). From there the milk goes through heat treatment to kill pathogens and then a homogenizer to re-suspend the cream back into the milk so it doesn't separate out into cream and skim again.
So if you want more calories in your milk then have at it, add some cream and shake it real well.
