So let me share a little story that happened to me recently.
Din's Curse just came out on Steam. Over at the Soldak forums someone asked if prior purchasers could redeem their game on Steam. The developer replied that he wasn't sure but that people who were interested could email him and he would see what he could do.
I had bought Din's Curse myself last year so I fired him off an email with proof of my non-Steam purchase of the game stating that I really didn't expect it, but that if he could manage to get a Steam key for me I would be most appreciative.
The next day I had a Steam key and was able to register it. THE NEXT DAY.
So either Arcen are making excuses because they don't want to say no and look like bad guys or they have some really bad contacts at Valve who are not being helpful and/or they just aren't trying hard enough.
I'm not saying they have to get the Steam keys for the prior purchasers in the first place, but if they aren't willing or able to do it for whatever reason just come out and say it. Don't string people along.