Last year I had a used the companion cert for a one way and later I managed to add a flight to it. I think I actually added a flight before. From what I remember I had HNL-OAK which I needed as final leg of big summer trip and then we decided to go to Maui in spring before and I added SMF-OGG (flew back on HA).

I currently have oneway booked again for HNL-OAK, again final leg of summer trip so will probably try to add a Mainland to Islands at some stage.