So not sure if anyone else is still battling their way through EA support with having issues on this, but I found a compromise when I fell into this same issue last night (IE bought from Steam and can't register). Due to the holiday it's currently on sale, so I just purchased the base game itself from the EA app for like $2. Logged in once through it to properly register, and then immediately logged in through the steam one and just had to log in to an existing account then. Yes, I realize it's not ideal to buy a 2nd copy of a game, but to me $2 was worth it to not have to deal with support after reading these stories.
I tried to solve it with EA, they've done nothing. But simultaneously I was contacting Steam Support and, after some back and forth, they finally managed to give me the Serial Key (CD KEY what ever). I think I will make a topic later trying to explain the process...
