Can I activate BMT Micro's key in Steam?

If I will buy GSB directly from this site through BMT Micro… Will I be able to use this key in Steam?
If yes, does it affects only core GSB or/and DLCs too?

So far as the main game, you should be able to register the BMT key with steam, as the BMT key is the “retail” key. I think this would only add the main game to your steam list and you would need to manually run the installers for the DLC, as discussed in this thread:
You’ll have to specify the install path manually, as the BMT Micro installer is looking for where the stand-alone version installs, not where Steam installs.

Edit: I realized that I’d only partially answered the question.