Indeed not possible IME.

However, I once added an infant on a domestic flight that was booked before the birth. The lady at AF Opera probably mixed-up things because I have been able to :
- OLCI (without the infant though ; he was "left alone")
- choose a seat on the very first row

When I came to the airport to CKI the infant, the AF agent had to change my seat because of safety reason. Apparently there is a limited number of rows whith additional oxygen masks. At least that is the explanation I received.