Once you have issued your boarding pass, there is no way you can change the FF number by yourself.

As stated above - lounge access is granted on the basis of being able to show a valid frequent flyer card of the relevant status in an eligible programme. If you are hoping to visit the lounge based on some other oneworld programme status, but earn miles in your BA account, put the BA number in the booking, and hand the boarding pass and your other status FFP card over to gain access to the lounge.