Yup, sure is. That's the same setup I have here at work. DVD OA + Steam A2. Works fine if you use a launcher like ArmA II Launcher
Though if you can afford it I'd get ArmA X. It's $60 for all
ArmA products directly from BIS
If you get A2 from Steam it's $20. Then you'll need another $20 for the DLCs eventually. So that's basically $40 right there. Think of the extra $20 as a tip to BIS for making awesome games. ;) You also get all the soundtracks and a bunch of extras too.
But if you just wanted to piecemeal it, adding A2 via Steam would be fine.