I'm really sorry, we can't participate in key trading. Purchasing from one storefront will only get you the DLC at that storefront.
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I'm afraid not, though I appreciate the offer. Steam doesn't want their keys being given away for a purchase on another site where they see no profit. There can be issues with your relationship with Steam if you do it too much. So, we can't make it a policy to give out keys in return for Itch purchases.
Maybe if you have a friend with a Steam account they could gift the DLC to you and in return you could get them something on Itch? I'm sorry we're not to make Steam easier to use or get around things like that.