Family of 6 (2 adults, 2 children, 2 infants) looking to travel to the States next summer. I have enough Avios to do an Avios + cash booking for all 6 of us. Or enough to do full Avios booking for 5. Having gone through the options, it would be cheaper to do the full Avios booking for 5, and pay cash for one of us (would need to be one of the children I think).

So my question is, is this possible? Can I do the Avios booking, then phone up and add another passenger paying cash?