You can read a good overview on Cushings including treatments here
http://www.marvistavet.net/html/body_cushing_s_disease.html
which should give you a good understanding of the disease.
If you want to try holistic care then the safest way to do that would be to work with a holistically trained vet who can monitor your dog in person and advise on which items may be right for your dog and safe to use. No one regulates the supplement type products so having a vet to help you is a good idea. This site may help you find a local holistic vet to work with
http://www.holisticvetlist.com/#search
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics