Not to over load you with "yes you can" info. Did this trip this past Feb 2004. The TGV station at CDG is in terminal #2 you can walk through the terminal if you are #2 by following the train signs (picture) if in another terminal take the bus marked terminal #2 a bit of a ride but it gets you there. Round trip second classe for 2 persons was 45.80 euro in Feb. All of the attendants at the ticket office in CDG speak very good English and pretty good English in Lille. When in Lille, there are two train stations approximately 300 yards or so apart. The Lille Europe is where you will arrive and it's about 6 - 8 blocks to the centre of old Lille, walkable down the hill when you exit the station. Be sure to check on the return ticket that you are not departing from the other station Lille Flanders, small mistake but a few hundred more yards to lug the baggage....

ciao, Charles