Extra character slots by adding another CD-key?

Do I get more character slots by adding another CD-Key to my account?
Even though I have all 3 campaigns unlocked.

If you have all 3 campaigns, you can't add anymore to your account. The only way to get more character slots (once you own all 3 games) is to buy them at $9.99 through the online store ("Enter Store" on the top right hand corner of the Character selection screen).

Adding two Keys for the same campaign will not grant you any additional bonuses. Infact, I'm not even sure if the game will let you do it.

You could however add Collector Edition keys to an account that already has the non-CE addition, but would only gain the CE bonuses, not more char slots.

In your situation, the only way you can get more character slots would be through the in-game store.

HTH

Edit: Beat me to it