To my knowledge it isn't possible. Here is a list of all games that can be activated for Steam with CD Keys, which I think is the only way to activate games.

However, you can add it to your Steam library. It won't download updates, initiate achievements, or anything like that, but your friends will be able to see that you're playing it.

You can try the CD key. I heard some publications enable it to be played on Steam.

Hello MoPiou.

Well, you didn't bought World of Goo from goofans.com, we are just fan based site. We don't sell it. You probably bought it from 2dboy.com and I afraid you can't register it on steam. The only way you can get it on steam is to buy it directly in Steam, or if you have Humble Indie Bundle 1 steam key.

Could you use the Sourcemods folder? I think it can be hacked to allow non-Source games to work in it.

Unfortunately, I no longer have either Steam or World of Goo, so I can't try it myself.

Hey, AP, do you still have your 2DBoy email or something you can get World of Goo back with? If not, I can give you the link to my HIB1 download page or something so you can get it again. Awful sorry about the hard disk troubles.

I think I can find the email, but thanks.