I like to sign in for beta games in my battle.net account. It says I have to register at least one game. If I want to add a game it needs the game key, but I just own CD-Keys of my Diablo 2 game, and the expansion Lord of Destruction. Is there a way to convert my CD-Keys to Game Keys?
The CD key is the key you need. Simply type in the key you already have, and the game should be added to your account. You can also download the game from the website after you've done this.
For Diablo II, I seem to recall it generates a new, longer key, and I think you'll need this longer key for the downloadable version - but as long as you use your CDs, you can just keep using the key you already have.
Yes, you will be able to convert those keys to Game Keys by signing in to Battle.net, and adding your keys here. The game key for Diablo II is different from the CD-key and is the key tied to the downloadable version of your game (after you link it to an account), but you'll still be able to use your existing CD-keys for installs off the discs.