Hi Jen,

Lounge access is granted only prior to departure in business class. As you will departing from CDG in Economy Class, you will not have access to the lounge ahead of this additional flight (unless you or a travelling companion has the relevant SkyTeam status that would allow lounge access).

It's surprising to read that "It's the only cabin available"; the J cabin on AF short-haul expands or contracts according to demand, so it would be very unusual that a J seat could not be provided (unless this was bought very close to departure). Could it be that you (or whoever purchased the ticket) bought this as a multi-city booking and requested that the short connection was in Y?