my ferrets don't eat rodents either but they probably would if they were given the chance to do so. I've read about people who let their ferrets play with their hamsters or rats and once they turned their backs on them the rodents turned into a delicious meal...
I prefer to give them special ferret food and sometimes high quality cat food. Every now and then, they also get some zuccini, because they absolutely love it.
Talking about the black footed ferret - I just saw them on tv yesterday. The used to be on the brink of extinction but I guess they now multiplied in the Californian deserts. We got lots of polecats over here, they look very much like my two sable ferrets... and they also eat rodents.
Ferrets for dummies really is a great book. I can also recommend it