Certain creatures that are considered carnivores are non-obligate carnivores and are technically omnivores (such as wolves/dogs). How different is the potential digestive ability of these species compared to humans? If they theoretically developed sapience and the intelligence for agriculture, would they be able to survive on the same wide variety of plant foods that we do (i.e. could wolves survive on wheat, rice, etc)
The foods people in the developed world typically feed their carnivorous pets (including dogs and cats) are not made primarily from meat. Instead, they are made from plants, specifically grains, such as corn, rice and wheat.
In order to get our pets to eat foods that are so different from the meat-rich diets of their ancestors, we add flavors to the food.
Check out this episode on the making of pet foods from the television series "How Its Made", http://www.youtube.com/watch?v=220PSFJWHao.
So, to answer your question directly-- yes, in places like the USA, domestic dogs and cats are already living on a diet that is almost entirely plant-based.