If your ferret gets sick, chances are your vet will tell you to feed
him softened food for a time while he recovers.  Even so, sometimes an
upset or recovering ferret will simply refuse to eat on his own.  If
that happens, a good thing to try is Gerber's Second Meals chicken baby 
food.  It's full of nutrients and water (though it's not a good 
full-time food [6.1]) and most ferrets love it.  Put a little on your 
finger and let your ferret lick it; if he won't try it, carefully
smear a little on his nose.  He should lick it off and eat the rest
from your fingers eagerly.  In general, ferrets like attention, and
they love to be hand-fed.  For a stubborn case, try letting another
ferret "raid" the sick one's food bowl in front of him.  Sometimes
there's nothing like competition to get a ferret to eat.

You can add Nutri-Cal, Pedialyte, medications, and so forth to the
baby food if your vet recommends them, and as your ferret's recovery
progresses, you can mix in portions of his regular food, moistened
somewhat, to gradually work him back to eating dry food on his own.

Sustacal and Ensure are sometimes recommended as short-term diets for
very sick ferrets, possibly in a mixture such as "Duck Soup" [12.8] 
but they aren't nutritionally complete and should never be used as the
only long-term food for a non-terminal ferret.  According to one
report, Ensure has the preferred flavors, but is also more likely to
cause diarrhea.  The best solution seemed to be combinations of the
two.