Can I register my physical copy with Steam?

[Tanshaydar]

It's not stupid. If you want it to be in your Steam library, you need to buy it from Steam since Amnesia DOES NOT use Steamworks.
You are talking like your copy doesn't work at all. Why pirate it while you have physical copy to install on your PC?

If you buy it directly from Frictional Store, money directly goes to Frictional Games.
If you buy it from another digital publisher, they'll have their cut.
If you buy physical copy then THQ takes its cut.

Frictional Games is the publisher in Steam, so THQ doesn't have any right or agreement on Steam version, you cannot include it in Steam library in any legal way.