We're having to feed a special diet to two of our cats who are ill. Arthur has runny poops, too. Yes, the pumpkin is a pretty common thing to ad to a dog or cat's food as a fibre source.
It doesn't have to be canned, although that is convenient. I baked a pumpkin from our garden and froze it in little portioned, pureed blobs. Unfortunately, Arthur won't touch it; won't eat rice either. The dog we had before this one loved the puree, though. He also liked pumpkin muffins. Canned food is easier to digest because it's sort of like a puree (less work for the digestive system breaking it down) and there's more water in it. You can get tinned foods that have added fibre, too. I will never feed a high-protein (grain-free) food exclusively ever again; it's what sent Mackenzie into renal failure. He's doing
sooo well now on a food that whose ingredient list would make a lot of people here cringe. I don't really care; results are results. Arthur, because of his condition, is grain-free still, though there's rice in his and it's not super-high protein. The canned fibre formula is too much for his system; one feeding firms his stools for three days. Effective stuff.