Is there anything that can be added to an expired box cake mix to leaven it again?
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You'd be adding eggs to your batter, right? That should do the job. If the mix does not smell stale, it should still give you the same result as a fresh box.
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How expired is it? Does it require you to add leavening, or is it included? If the latter, and the leavening has expired, we could help you if we knew what volume of dry ingredients you have.
Cake will probably not rise like it would if the box was fresh.