This is the Flying Blue forum.

For questions related to the SAS app/website and the Eurobonus programme, you really should be asking in the SAS forum - where those who use the Eurobonus programme, and are most familiar with the SAS app/website, are more likely to be found.

But I can answer this for you - you generally cannot add/replace the FFP number in a booking while you are logged in using the credentials of a completely separate other FFP.

I would not expect that, when logged in as a Eurobonus member, that you would be presented with the opportunity to "add" (surely, replace?) a Flying Blue number on the booking. Just as, when logged in as a Flying Blue member, neither AF, KL nor Flying Blue would present you with the opportunity to "add" a Eurobonus number.

Once your Eurobonus number is associated with the booking - as must be the case, given that you are logged in using your Eurobonus number - you cannot add a Flying Blue number. You would have to seek assistance from an SAS agent in order to remove your Eurobonus number (and eventually add your Flying Blue number).

Please be advised, though, that changing numbers is a risky business; the miles will frequently end up in the "wrong" account. I would advise you, going forward, to consolidate your flying in one programme or the other - for the vast majority of flyers, it doesn't really make sense to maintain two (or more) different frequent flyer programmes from the same alliance - simply because it's easier, for most flyers, to obtain status by concentrating all of that alliance's flights onto a single programme; additionally, by occasionally "topping up" two different FFPs, it takes you longer to reach a usable amount of miles in either - rather than, again, concentrating all of that alliance's flights into a single FFP