So you didn't buy your game from Steam but you want to use Steam's services for free? :P This is an issue you need to take up with Steam/Valve. Steam allows for shortcuts to your games to be launched through Steam, that you can do with any game, but as far as "activating" the game goes, see above :)
Yeah, so in other words, we are being punished for trusting Origin and buying the game through it after it was removed from Steam.
If i got this correct then probably Steam gets a share of every game sold through their store, and all the Crysis 2 copies that has been sold through Origin, Steam did not get any benefit out of them, and so it does not make any sense for Steam to allow the activation of the copies sold through another service.
But if that was not the case - as i could activate on Origin my Alice: Madness Returns' key that i bought through Steam so there might be a chance that it could work the other way around - and if the game key purchased from Origing can be activated on Steam with no problem, then i think that the decent thing from EA, Crytek and Valve to do is:
To allow the people who bought Crysis 2 on Origin to activate their Key on Steam and pay only for the DLC to upgrade their game to the maximum edition.
Or to allow the upgrade to the maximum edition on Origin with the price of the DLC only, and do the same with Steam and allow the upgrade to the maximum edition with the price of the DLC only.
Also convert the players who bought the Limited Edition to the Maximum Edition since as far as i've read they are both the same thing.
And i'm not going to be greedy and ask to have the DLC for free, but i still would want to be able to activate my game key on both Steam and Origin.
Cause seriously, i don't think we deserve to be punished for trusting your services at first place.