If it is true Pred belly and all else is being done (fat, not fluid
which IS something to have the vet check) then only exercise which
will also help some with the rate of muscle loss from steroid use.
 
If the ferret has insulinoma which still have room for medical adjustment
you can try adding Diazoxide to allow you to reduce the Pred levels.  If
you are giving Prednisone instead of Prednisolone/Pediapred you can try
switching to Prednisolone because that often works better, and thus may
allow reduction in dose though that might not effect the pred belly -- I
don't know which aspect of the pred leads to pred belly.  (The liver
converts Prednisone to Prednisolone so if the liver is not working as
well as one would like the Prednisone won't work all that well.) If
insulinoma has surgery been discussed?  Nor do I know if an alternative
steroid would be useful for your little one.
 
If the problem is something else like lymphoma or carcinoma or if heart
or kidney disease may have begun consider the possibility that you may
alternatively be seeing ascites and get to the vet rapidly for a
check-up.
 
Sounds like perhaps it's time for a vet check-up and discussing such
things to maintain quality of life.
[Posted in FML issue 4739]