I started using Keeper's Mix a few weeks ago to get my bitch back into coat and condition after pups, and it does say on the tub and on the website it is perfectly safe to add to complete foods. This is what it says:
Keeper's Mix contains a blend of nutritious herbs, carefully formulated so that their individual properties provide health and vitality to all the major organs without any risk of overheating the system or unbalancing the diet, even when given to animals that are fed complete diets.