In order to redeem the codes, you need to log in to your Steam account through the Obsidian site. So no, I don't think you can do it that way. You'll have to purchase the game first, then do it. If you don't know for sure please don't spread misinformation. The Steam login is used to redeem packages directly to your account ignoring any requirements. You can check what has been redeemed on your licenses page. So to answer the question: Yes, you do not need to own anything to receive the items.