@Serafielus - I'm sorry, if EA Help can't add the game without the CD key, you probably need Steam support to see if they can help you find the key. Then, if you can get the key, EA Help might be able to add it to your library (you can always try a different language support, too)
I know that people have had difficulties redeeming old retail keys in Origin, too, but I hope the newer digital downloads have a product key that works. I hate to be THAT person (sending you back to Steam), but I can't see another way to get a key. Since Steam is where it was installed, they should be the ones to know where the key is. I've done a lot of searching for answers, and found very little help. This post says that Steam uses one key for every install, so you can never register it in Origin. This one has an idea that I wouldn't recommend unless you are very knowledgeable about computers.
Wish I could be more help. Good luck! 🤞
