Actually, if I remember correctly Ferrets are a true carnivore with a 4 hour digestive system. A cat, for example, would be a true carnivore with a 6 hour digestive system. .. so there have been instances where ferrets do not do well on cat food because they cannot absorb the nutrition they need from it before it's passed out of their system.
Most pet shops carry a blend of food for ferrets, but a high quality cat food should be ok in a pinch. You can try a wet mixture of canned food as well.
Good luck!