according to this page on the official steam-works website [https://partner.steamgames.com/doc/features/keys], only 5,000 keys can be generated before they have to be reviewed on a key-to-key basis. and seeing as only 13 days after this game released on steam, it reached 1,000,000 sales, i doubt that would be an option.
i've talked to mike. he's genuinely a really nice guy. i feel like taking money from his earliest supporters just to be greedy is not in his priorities.
also, you do know how hard mike has been working these past few months, right? imagine making an indie game, that pops off, and then you get bombed with questions asking for multiplayer. when you do make it, people get mad because their version doesn't have it. you doing this is just making this whole situation worse.