Yes, in theory you are correct BUT
Apparantly, I've heard that the FNA has to be done in one sector service not via anywhere as they initially did it or they will lose a lot of business/route due to political reasons.
ALM can't be reached directly on a 321. Therefore it would need a tech stop somewhere. Can you imagine the number of passengers bmi would have going to somewhere like ATQ (if true), who would be prepared to fly 3 sectors to get there? Not many when there are other carriers flying wide bodies in 2 sectors.
So that returns us back to the original question, how would they manage it without an extra airframe?