Further to what has already been written by other posters here, even if you had created a Flying Blue account, your reservation would not magically appear nor would there be any risk that the FB number would replace your DL SkyMiles number.

Since there was never such FB number when you initially made your booking (you input your DL number instead), your reservation was never associated to any FB number.

And to your last remark, even if you create a FB account and while logged in it, you add the reservation with your DL number, it would not necessarily pollute and override your DL number with your FB number.

Looking up a booking with the "Add/Look Up an Existing Reservation" merely displays that reservation locally in the environment where you look it up: for example, if you decide to look up an external booking (yours or any stranger but for which you know the PNR and Last Name) on the AF app, you won't see that booking on the browser.

Looking Up/Adding manually a reservation for display is limited to the local platform where you view it and independent whether or not you are logged in: the booking is "insulated" and doesn't interact with your account.

The only way where there might be some pollution is if you purposefully change/edit the FFP number (online, at the airport/gate etc...) where the system can get confused between all the different numbers successively entered.

Even then, in my experience, I have rarely had those issues, despite switching between my FB and my DL accounts. Here is an example where you would think there would be a problem:

- I make a booking on DL while logged as a DL SM member, so that all my info feed the PNR (incl. my Known Traveller Number, Passport etc...)
- my booking shows up in my DL app with my DL number
- I switch my number into FB
- My info get preserved (passport, KTN etc...)

The miles are properly credited to FB without any incident. But granted, this is DL IT. The reverse with the AF IT might generate issues...