That should make absolutely no difference - you should always be able to "add" your number to the bookings regardless of whether the ticket actually earns miles or not (so that, for instance, those members with status can benefit from their status perks).

a) where, and how, exactly, are you trying to "add" the FF numbers? Why didn't you do this when buying the tickets, and filling in the names of each passenger?
b) If you're trying on KLM.com, try on airfrance.com instead. Or have you tried at the Korean Air website? There is absolutely no reason why you should have to wait until check-in to be able to add your numbers.