Don’t mess with award tickets - you just can’t combine awards and add a leg. UA just doesn’t work that way. You may think they should be able to but, UA DOES NOT combine itineraries - period.

inter-island airfares are cheap nowadays, thanks to presence of WN - just buy a ticket. If you want two itineraries on same record, you’re basically asking UA to re-ticket at current mileage levels using current inventory.