I have an international flight with American (booked through BA). Am I able to add additional checked bags after I have completed online check-in when at the airport, as I was unable to do so online while checking in?
They will always gladly take your money (and your luggage).
Just make sure to be early enough at the airport; there are deadlines for the luggage dropoff, and they are earlier than the 'just-walk-to-the-gate' deadlines. Typically, they close hard an hour before the flight, but check yourself if you plan to cut it tight.