You can always change your itenerary as it becomes available. Probably costs $75-150 to add a city or whatever, but waived for elite members. What you can do, esp for UA/AA, is to do your trip and add Hawaii for whatever dates are available. Both allow free date changes outside of 21 days (AA all date changes are free, I think), so when your dates are open, call and get them to change for free.

Also, outside of holidays, Hawaii is often available a few months or even weeks out.