Most of the islands "in between" cannot be reached using lowest fares US-NZ. Best to view it as a separate trip and not a "free stopover along the way". Sometimes FJ has cheap fares via NAN (esp. if you buy separate tickets as the ex-NAN price can be much cheaper), and TN can also be cheap(er) at times via PPT. Air NZ also has "island hopper" fares at times, which is a great deal but typically double the price of just visiting NZ. Finally there are great tropical beaches in the north island of NZ, maybe better than you get at those islands you want to visit. Personally I'd spend the time/money to go to the more remote parts of NZ (such as 90 mile beach).