There's always a MOB waiting for 'Ohana so I would say it would be best to keep searching for a new ADR for 6 people (don't cancel your old one until you find a new one.)
It's not a matter of pulling up a couple extra chairs, it's a matter of restaurant capacity and fire codes. And since 'Ohana is frequently packed, I would say it's going to be tough to show up with extra people and get seated.
Good luck! If all else fails, you can show up with your extra people but have a backup plan just in case. Would you all leave if turned away or would 4 eat there and 2 eat elsewhere? It's good to know ahead of time.
Edited to add: Kona Cafe is right next door. You can probably get an ADR there for two people around the same time as your 'Ohana ADR. That way if 'Ohana can't handle your party of 6, then two of you will have somewhere nearby to eat.