Yesterday I bought a ticket for RER B train to CDG Airport. I bought this ticket on Chatelet Les Halles and didnt mark it as I would like to use it tomorrow. It is possible to use this ticket for Metro and start the journey from from my hotel destination (Argentine station, in District1)? When I mark the ticket in Argentine, will I be able to enter RER B with the same ticket on Chatelet Les Halles? Thanks!
Yes the Paris RER train ticket is valid for the Metro as well. Any RER ticket which has either the starting or ending station within Paris center (i.e. any Metro in Paris or RER station in Zone 1) will be good for Metro transfers anywhere along the route of travel (beginning, middle, end).
Using the Billet Origine Destination (RER) ticket, you can enter at Argentine Metro station, take the Metro to Chatelet les Halles and transfer to RER B to CDG Airport.