You can book this kind of ticket with United miles, however you get them. For example, if you do a transfer from Ultimate Rewards that works too. The key question: is this redemption the best value for YOU? That's mostly a factor of how else you would use the points/miles you have. This is an option - one of many.

Even without the miles, you can look at some example dates - not the ones you want, since they're still too far out to book, but other dates, to get a feel for the system. When you get to the booking page, it'll tell you how many miles are required and then remind you that you don't have enough.

Keep in mind, if your plan all along is to book SFO-OGG using 20k UA miles and ITO-ROC using another 20k UA miles, then it's the same price to add on the ROC-SFO to the beginning. There's no sense in using A+ credits in that case.