Sorry I don't have experience with Lufthansa specifically, hopefully others do. I just used FBC to book United flights ahead of final payment and although they didn't automatically show in my United app, I was able to add them using the airline confirmation number in my FBC e-mail. One cruise I did by phone with my PVP, one I did myself on the website. I then made a change to the first one with FBC subsequently. I was able to go in on the United site and add the flight to my profile then pick seats. FBC also offered to do it with me over the phone but I told them I preferred to do it on the site myself looking at the seating chart.