Yes you can however if you have a Baec account it will default to the BA ff number as you booking with avios . I have done this a number of times waited 5 mins after making the booking and then going into finnair website MMB and finding the PNR there and then changing my FFP to QR saving it . Usually within 5 mins can go back to BA MMB and number has changed and the recognises status and offers free seat selection. I think you can also do same with other OW partners by Finnair always works for me