I have used baby cereal with my crew a few times in the past. Essentially, the powdery flaky stuff that you can get in the baby food department. Just sprinkle a pinch over the regular kibble food. Doesn't require preparation or fancy storage. If you use some other form, you would probably need to grind/crush it into essentially a powder, or the hedgehog very well may pick through and around it. The dry baby cereal tends to cling to the kibble, making it more difficult for the hedgehog to avoid. I haven't noticed with my crew that it is a NEEDED item, though I guess there would be circumstances where the added fiber would be useful. I can't think of any reasons why it would be harmful in any way, so it shouldn't hurt if you decide to use it. Just make sure that the hedgehog doesn't quit eating after it is added, and monitor poops for major changes in consistency or amount.
Becca