Put both foods in a container, so the ferret food will have the smell of
the kitten food.  They go by the smell of the food.  Start with a small
amount of ferret food and gradually increase the ferret food.  I had a
situation like yours and this worked with him.
 
Phyllis and the Wigglies: Generosity, Grace and Joey (Joy)
In Loving Memory: Faith, Hope, Constance, Charrity and Bandit
[Posted in FML issue 4565]