Someone else will need to weigh in on the Fiji NAN-BNE. But your strategy is similar to what I did for June. I booked SFO-MEL. Then one month later added the domestic Australian leg on QF I wanted. Another month later (today in fact) added the US domestic leg. The main rules are you can add with the stopover option so long as you stay on the partner or AS metal (includes Skywest). I personally did it over the phone with reservations on the Gold phone line. Most agents were quick to pick up on what I was doing and there might be some minimal taxes due when adding.