Old thread is old, but I'm bumping it because I was just going to make a threat about the same damn thing otherwise.
Anyway, if you want CoH mods to work with steam, here's how you do it. When you install mods like Blitzkrieg, it has an icon on the desktop to launch it. Now, if you open the shortcut properties, you'll see something like this:
"C:\Program Files (x86)\Steam\steamapps\common\company of heroes\RelicCOH.exe" -dev -nomovies -mod Blitzkrieg
The bit in bold, pay attention to it. Copy that. Now go to your Steam library. Add a shortcut for RelicCOH.exe (like you would on any game adding to steam) and open the properties of the shortcut.
Now, there is a section that says "Target" and in "Target" it has;
"C:\Program Files (x86)\Steam\steamapps\common\company of heroes\RelicCOH.exe"
Add to the end of it;
-dev -nomovies -mod Blitzkrieg
That way it looks just like the shortcut from the icon on the desktop! Enjoy playing Blitzkrieg with Steam! Also, come play with me some time. There's a certain thrill in watching hoards of AI players trying desperately to swarm me while I chew them into giblets with my iron wall of defense.
Or you can do it Duran's way if you don't want a shortcut specifically for Blitzkrieg.