Passage : Retail -> Steam

[RaiseoHitsu]

This is the problem :
I've bought Arma II Retail near my local gamestop, great game, 40 Euros, enjoyed it, then forgot about it.
Now, the expansion got my attention and the next steam sale that expansion is going to be mine, still, my Arma II copy is retail, perfectly 100% legal and genuine (yes, I can prove it, I have my serial key) BUT the serial key doesn't work on steam (figures).
I wish to have Arma II on steam, not only for the auto patching, but all the goodies steam as a platform has to offer, plus I wish to buy the expansion and "expand" my game.
My ultimate objective is to play (Yes, I admit it, part of me just wants it for) the ever popular Day Z mod, which requires Combined Operations to work and I fail to see how would that work if I had my vanilla game installed normally and the expansion installed on Steam (and even if that would work, I still would like to enjoy the original Arma II on steam).
So I ask for help in this matter, any mod, admnistrator, how could I do that?
Thank you for you help in advance.
EDIT : Oh I'm really sorry, it seems it has been already asked So I read every version of arma 2 is compatible with every version of OA, right? Then there is no problem I guess, still I would prefer to own Arma II on steam, if it's not possible to do this sort of "passage" then it's ok.

[Leon86]

save yourself some trouble and just buy the retail arrowhead, it's on sale now
https://store.bistudio.com/arma2-operation-arrowhead

[Placebo]

Retail non-Steam Arma2 cohabits perfectly with Steam (or other retail) OA to then form CO, but obviously if you want Steam A2 you must now buy Steam A2, as mentioned there's a sale ongoing with our store and at some point Steam themselves will have a sale so the simple answer is pretty clear, I'll close this as there seems to be nothing left unanswered, of course if that's wrong feel free to PM me