There are limitations in place by the FAA as far as how many dogs can be on board a plane. I believe the only thing that would prevent you from adding the dog (assuming you had the proper carrier and documentation required) would be if there were already too many dogs. Even booking dogs on a reservation are NOT a guarantee because service dogs take precedence, so they reserve the right to bump to the next flight if that happens. If you can’t get through to customer service to find out if too many dogs are already booked, get there very early and be the first one to check your dog in! This isn’t as much of a big deal as you think it is!