I'm not sure about natural, but there shouldn't be a problem feeding chicks to your Ball if he'll accept them. He might not accept them, though, if he as already eaten rodents. Balls are imprint feeders which is why many people have problems switching to different types of feeders.

One more thing to keep in mind is that rodent fur is a much better binder than sparse chick feathers which means you may have more of a mess (runny stools) to clean up when he defecates.