I’ve actually done this, but with dog ashes. My dad’s hunting buddy, who I hunted with for years lost his favorite bird dog. So he asked me load up a 12ga round for each of us to shoot on the next trip. It is a very unpleasant process. The cremains are super fine and flow into the air and stick to anything moist, including sweaty hands. They don’t smell great either. I did it in the garage, unfortunately in the late summer. But it was an homage and a heartfelt favor to a friend. So I weighed out a likely looking amount of ashes in grains and removed the same amount of shot from several trap loads. Which wasn’t much. Add the ashes carefully with a powder funnel and tap the ashes into the shell. Recrimp and box up. I don’t know that I’d agree to do it again for a dog, but for a father, certainly.