Maybe I misunderstood you. It sounded like you planned to book this way:

ROC-SFO for 8 A+ credits
SFO-OGG for $176 cash
OGG-ITO for 7k Hawaiian miles
ITO-ROC for 20k UA miles

It's not sure 8 A+ credits + $176 + 7k HA + taxes is better than 20k UA + taxes. It might be, depending on how you would otherwise use that currency.

You can easily book the combined ticket on United. Search for one-way availability on each leg of it and write down the flights you find. In other words, search for ROC-SFO, then SFO-OGG, then ITO-ROC. After you have dates with availability, switch to a Multiple Destination search and pick those same three legs with the dates you found. Usually, the same flight options will appear and, at the end, it gives you a single ticket booking. Occasionally, you won't see the same flights, but you can call to book in that case.

Out of curiosity, I tried to book the four segment trip with the OGG-ITO included. The website won't do it. So you'd have to book that 5k ticket separately.

(Note that the OGG-ITO may be 7k Hawaiian miles, but it's definitely 5k United miles booking onto the Hawaiian flight.)