Are you booking using the Free-at-sea BOGO offer or NCL's Premium Air?
BOGO - you can upgrade at your cost once the flight is ticketed, IF the airline allows it with the fare that was negotiated. Deviations to arrive or depart up to 2 days before and after the cruise are allowed. Here is the T&C's that cover the BOGO - https://www.ncl.com/termsandconditions/promotions
Premium Air - the wording appears ambiguous but it MAY be allowed. right from this page - https://www.ncl.com/freestyle-cruise/premium-air
The types of customized flight itinerary requests handled by the Premium Air Service Desk are date deviations, nonstop service, specific airline, specific flights or specific departure time, stopovers, upgrades to business or first class and gateways not offered in the Air Sea program.