Cream contains fat, but no calcium. If you dilute cream, you don’t make milk, you make thin cream.

If you’re worried your milk doesn’t have enough fat in it, add cream to milk. Dont dilute cream with water.

Really puzzled by the description of pasteurised milk as “dead white water”. It’s milk, with calcium, fat and other nutrients. It just doesn’t have bacteria. If that bothers you, culture it. Personally I don’t find pasteurisation at all insulting. I’m glad that tuberculosis is no longer endemic,which it would be if we hadn’t started pasteurising milk.

http://www.sophisticatededge.com/how-to-make-buttermilk-from-milk.html

“Using Milk as a Starter

If you don’t already have an active culture from buttermilk, you can make your own. For this to work you will need to use fresh raw milk, which means it has not been pasteurized. Be aware that the FDA warns that raw milk can pose a serious health risk.

Let 1-cup fresh raw milk sit at room temperature for several days until it clabbers. Use ¼ cup of this newly clabbered milk and add a cup of fresh milk. Let it sit until it clabbers. Continue this procedure until the mixture reliably clabbers in twenty-four hours. Once it does, this mixture can be used confidently as a starter for larger batches. “