You can recharge your passes but most likely your
passes don't cover the trip from CDG... you can get return tickets there for 8.70 euros each.

GiuliaP - unless you plan to travel way outside Paris into zone 5/6 (like to CDG) every day during the validity period of your Navigo pass it makes no sense to purchase the zone 1-6 pass. For the trips into town from and back to CDG just buy a ticket, then use your zone 1-2 pass for hopping around town.

Thanks for the info! So we only fly in to CDG but then we leave by Eurostar at Gare du Nord. So should I only buy a one way ticket, or does the same line that goes back to CDG pass by Gare du Nord as well warranting a return ticket? I'm horrible at public transportation, I don't know why the metro is so baffling to me!

RER tickets are all "one way", they are similar to Metro ticket that way.

Gotcha, thanks!