If milk is not native to this Island, then just do without.
Coconut milk is far more superior in nutritional quality and is in greater quantity.

As for herding cows on this Island, it's almost sinful.
The grounds were not made to handle large grazing animals such as cows.
It wold be quite a waste of grounds for a product that is so unnecessary.
That's why Island countries usually have to pay enormous prices for any products related to cows (beef, dairy, etc.).

There are many alternatives though. Coconut, goat, and even buffalo which are still used on farms for plowing.

They should just stick to a more local diet which I am sure is plentiful and not adopt European/American influenced diets that require importation.

Cuba has a very diverse plant, animal and sea life that they have access to.
There really should be no problems.