Can I turn skimmed into whole milk?
Skim milk is whole milk with the cream removed so it can be sold separately.

Simply add heavy cream. The ratio is about 1/3 cup of cream per quart of skim milk.

Keep in mind that whole milk has been homogenized. Your cream has not and will separate out again if left undisturbed. This is not really an issue for cooking just if you plan on drinking it.