We've just booked disney and just added baby with due date and infant for their first name. Spoke to disney after and they said under 3s are free so could have just added when baby was born. Worth just giving the company a call. May be different for flights.
I went to Florida and flew with virgin airlines they wouldn’t let me add the baby till she was born and knew the name. I’m assuming it may be different depending which airline x
Call the company and they can do it for you. You will then need to contact them when baby is born with the info. I've just done this for a holiday in January.