Can coconut cream be transformed into coconut milk, or the other way around? For example, will adding water or cow milk to coconut cream make a usable coconut milk? Or by reducing coconut milk, can I get coconut cream?
Short answer: you can let coconut milk separate and skim it to get a little coconut cream, but can't convert coconut cream back to coconut milk. Diluting the cream just produces something runny and disgusting.
Long answer: Coconut milk is made by grating coconut and running very hot water through it to extract oils and flavor, then straining out the coconut pieces. It is a mix of water soluble parts and fats, and can emulsify just like normal milk or thicken sauces similarly to milk cream.
If you allow coconut milk to sit, the fatty part will rise to the top. This part is skimmed off to make coconut cream, which is much richer in fats. It cooks very differently because of the higher fat content, and does not emulsify or thicken well; however it is quite delicious!
If you're looking to substitute for coconut milk, but only have coconut cream, the best substitution will probably be a little coconut cream + a lot of heavy (milk) cream.
Note: Some people refer to the liquid contained in fresh coconuts as coconut milk, but it is more accurately called coconut water. This confusion has ruined many a recipe!
The coconut milk is refrigerated and allowed to set. Coconut cream is the thick non-liquid part that separates and rises to the top of the coconut milk.
To get the other way you can surely add milk or water. This will not give you the same exact thing as coconut milk, but it would be ok as a substitution for a dish.
For converting coconut cream into coconut milk
There seems to be a lot of disagreement, so you'll probably need to experiment (like #5 below suggests).
Answer 1: BobMcGee says you "can't convert coconut cream back to coconut milk".
Answer 2: In my experience, I've always added 1 can of water for every 1 can of coconut milk, and it works great for my purposes.
¾ cup water per 2 tbsp coconut cream
https://amzn.to/2ItFPHp says a 2 tbsp serving size of coconut cream weighs 30g and that a 13.66 fl oz can (403 mL) has ~13 servings.
13 × 0.75 cup = 9.75 cups water per 13.66 fl oz can of coconut cream (i.e. 9.75 c water per 1.7 c coconut cream).
I personally expect this approach would taste far too watery.
The ratio for coconut milk from canned coconut cream is: 1 can of coconut cream (5.40 oz) and 2 cups of water.
If we assume that Willamette meant to write "5.40 fl oz" instead of "5.40 oz" (i.e. a volume measurement rather than a weight), then scaling* her recipe leads to: 5.1 cups water per 13.66 fl oz can of coconut cream (i.e. 5.1 c water per 1.7 c coconut cream), which is ~1.9 times creamier than the Potamus recipe, but still far less creamy than I prefer.
*13.66 ÷ 5.4 × 2 ≈ 5.1
Answer 5: https://www.wikihow.com/Make-Coconut-Milk-from-Coconut-Cream does not provide a specific ratio and instead says:
The amount [of water to mix with coconut cream] will depend on the consistency you want. It's best to add small amounts and build up rather than to overdo it.