Any quality brand of commercial ferret or cat food you choose will have all the vitamins your ferret needs, so you have no need to vitamin fortify your ferret's food. How do you know if the commercial food you use has enough vitamins in it? Check the list of ingredients. Your ferret's food should:
If your ferret's food meets those requirements, then the quality and the quantity of the fat and protein in the food will be enough to supply all necessary vitamins.
If your ferret's food does not meet the guidelines above, you could vitamin fortify it yourself. However, it's easier just to switch to a better brand of ferret food. If you're intent on making your ferret's food instead of buying fully prepared commercial food, at least make a reasonable compromise. Buy a base food, like Purina Carnivore Chow, and add beef or poultry to it for the proper protein and fat required. The combination will have the proper amounts of vitamins, and have the advantage of providing fresh meat and fat for maximum nutrition and minimum spoilage.
If you must vitamin fortify your food, use a commercial supplement after consulting with your veterinarian. And be careful not to give too much, since supplements contain fat-soluble vitamins like A and D, which are toxic in large doses. (A and D toxicity manifests with just 4-5 times the minimum daily requirement in humans.)
Most of these supplements also include fat, so adding them to food will make food spoilage more likely - uneaten food will have to be discarded quickly. Although some supplements come with instructions to coat your ferret's food, avoid doing so as you would be giving your pet an excessive amount.