I am asking for a colleague.

He wants to fly France --> USA and his choice of flight is restricted. He has to take Air France and the price for a checked baggage is very high (1100€ for a 700€ flight).

He wants to know if he can instead book the flight with no luggage, then arrive early at the airport and book a checked luggage on the spot.

I have done it on a RyanAir flight and it cost me 20€ at the airport, last minute. But it was a much shorter and cheaper flight.