What do i do if my ferrets only eat cat food?

I just bought two ferrets yesterday from some people. They said they fed them cat food along with vitamin supplements, but Ive read from multiple sources that ferrets should NOT be fed cat food because they have different digestive systems. I bought them ferret food, but they wont even eat it. Whats the best way to wean them off cat food? Is it okay for them to eat cat food? Any help ASAP would help.