They may have trouble if stale means fungus as well. If fungus not an issue, then you may be able to enhance value by adding a little chicken feed. We have been using non-medicated chick grower per directions of another party at 25% of total mix.
The best food for them is wheat bran. Many use other grains or a mix. Some use oats but that makes it more difficult to "sift" them out. If you use any processed food (chicken feed, dog food, etc) make sure to read the label. If it contains DE (Diatomaceous earth) it will kill your meal worms, exactly as it's designed to do (kill bugs) when added to feeds.