Step 3. Serve your dog a little oatmeal, rice or whole-grain pasta. If your dog already eats a kibbled dry food diet, don't feed much of these; it's already getting a lot of grain. However, if you want to add fiber to a raw or home-cooked diet, grains add both carbohydrates and fiber.Step 4. Mix some wheat bran into your dog's food.Bran is very high in fiber, so just use a sprinkle at first.owever, if you want to add fiber to a raw or home-cooked diet, grains add both carbohydrates and fiber. Step 4. Mix some wheat bran into your dog's food. Bran is very high in fiber, so just use a sprinkle at first.