The book also says to limit the fruits because of the high sugar in them. They can harm ferrets with insulinoma. Raisins are great treats, but shouldn't be used as whole meals. They can also have 1-2 small pieces per day of melon, banana, apple and papaya.
You can give them veggies, but only cooked ones because uncooked ones may get lodged in the intestinal tract. Some veggies you can give are green pepper, broccoli and cucumber (with no skin), chopped into small bits. However, veggies have little nutritional value for the ferrets (remember they are carnivores) and they'll come out the other way basically undigested.
You can also feed them cereal treats with very low salt and sugar (book recommends Cheerios and Kix), and they should not be fed more than 2 pieces a day.
Hard boiled or scrambled eggs are greatly appreciated, but avoid uncooked eggs, especially egg whites, because they contain a substance that bonds to the biotin in the ferret's diet and is harmful to them.
You can feed them peanut butter, but only the smooth type, and not too much of it because it can cause diarrhea.
Also (yuck) you can feed them, as treats, COOKED chicken liver and hearts and beef and turkey. Avoid processed meats (such as lunchmeat or salami) because they contain too many salt and additives.
You can try giving it a bit of dairy products, like a small piece of cheese or a bit of cream, but some ferrets are lactose intolerant. You can determine this by checking their poop - if the eaten dairy product comes out as a smelly liquid he's probably sensitive to lactose.
Other things to avoid are caffeine products and alcohol or high-sugar drinks, seeds and nuts (indigestible and can cause intestinal blockage), chocolate, sugary foods and salty foods).
A good fatty-acid supplement that they seem to love is Linatone (for cats) or Ferretone (for ferrets). These are great for thw upcoming winter months when the ferret's coat and skin may get a little dry. The author feeds them a few drops once or twice a week. Pouring a small amount of the supplement over the food is OK, but this will spoil the food over time so I recommend giving them a small amount of food with the supplement so they eat it all up and no food goes to waste. Supplements with vitamin A are harmful if used in large amounts. She also feeds them Nutri-Cal and Ferretvite, about an inch of it, daily. It's a great calorie boost which I think would help you underweight ferret, but remember it's very sugary (it kinda tastes like molasses), so you should feed less than this amount to insulanoma patients.
Source: Ferrets for Dummies by Kim Schilling
OK...I need help from Ferret people. I will be picking up a Ferret to foster while she gains some weight, and I know practically nothing about them. They are giving me a cage, and I have a corner litter pan, food dishes, water bottle and towels. I assume she will need some kind of cozy or hammock. They told me just to feed her kitten food, but I am going to the store before I bring her home, so what else do I need to get? I know she's been vet checked, but are there any things she might pass on to my piggy? Do I need to keep them in seperate rooms? 
