Yes, you can thaw, portion and refreeze meats intended for cats. To keep nutrient degradation as minimal as possible, I only defrost it as much as is necessary to break or cut the food apart.
To BotanyBlack's second point - there are different comfort levels with that kind of food acquisition behavior. While some folks are comfortable with the practice, I wouldn't (don't) feed my cats anything I haven't personally sourced myself (i.e. no second-hand, freezer-burnt, unwanted, etc. meat).