Ehh harder than dogs, easier than cats usually. Just have to be persistent and convince them meat is food, I've swapped 5 ferrets, all adults to seniors from kibble to raw. Got a new boy in Dec 2012 and had him on raw within a month. He was underweight and his coat felt super coarse and gross, now he's a fattie but so soft. Mine eat mice, quail, rabbit, chicken, turkey, goat, duck, grouse and a few other proteins less often.
Hannibal my newbie eating turkey ribs
mazuri ferrets looks like junk btw
http://moredooks.herobo.com/search.php?chart=ferret