Actually you're in luck, Relic are in the process of moving Dawn of War 2 and Chaos Rising over to Steamworks now. The third expansion, Retribution, already uses steamworks and the original DOW which used Gamespy has already been moved to Steamworks as well.
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Not sure when it'll happen, but it can't be too far off. A quick glance at the steam forums reveals that apparently the achievements for DOW2+Chaos Rising popped up on Steam in late April so it's only a matter of time now.