I recently booked two BA flights using Avios but wanted to add my AA number. I used the RAM site to pull up each booking using the booking reference and remove my BAEC number. Subsequently I was then able to go into each booking within my BA account and add my AA number. I would see whether you can do something like this.

(Previously I had done this manoeuvre on EY but couldn't get it to work there, nor on RJ.)