It's not even an LH-group thing: LH is on Amadeus so you can edit FF details on most Amadeus airline websites. A lot of BA FFs use the Finnair website to edit FF details, as the BA website won't let you change FF numbers once you've added them to a PNR. You should be able to edit LH bookings on the Finnair site as well. Whilst I've never done that myself with an LH booking, last year I was able to bring up a LO (who is also on Amadeus) booking on the Finnair site to modify APIS details. This is the main reason why I'm looking forward to the AC move to Amadeus...