Visiting Paris next week, staying near CDG, looking at using RER to get into/out of central Paris and walk around within.
I know that RER tickets are based on station-to-station, but the cost is set by zones. Is it possible to either enter or exit the RER system at a different station (but within the same zone) as that on the ticket?
Basically I'd like to buy a carnet of tickets to get to ie Chatelet but return from ie Champs de Mars. If that's not possible, would it work in reverse, so I'd buy a carnet from Chatelet to Expositions, but then use some tickets to come back into Champs de Mars.

Anyone?

[and I tried searching both here and on google but as soon as you mention 'carnet' all you get is metro/t+ references...]