I will be spending a day in Paris with a friend, arriving by train into Gare du Nord in the morning, and flying out in the evening. However, my friend leaves from Charles de Gaulle, and I leave from Orly. I'd like to accompany my friend to CDG. On my return trip to Orly, I'd like to avoid Orlyval and take the scenic route via the T7 tram.
To make these two trips, it would cost €10.30 on each leg, plus €1.90 for the tram (€22.50 total). However, this doesn't afford us any additional trips during the day. A Mobilis ticket -- which would give unlimited travel but not allow travel to CDG by RER -- costs €12.40 for 4 zones. (Edit: a 4-zone Mobilis allows travel to Orly via tram T7, but not to CDG as Mobilis tickets cannot be used for airport connections -- for this reason, a 5-zone Mobilis ticket would be an extra expense for no gain).
Is it possible to use a one-way ticket from an RER station in zone 4 to CDG (or vice-versa, e.g. Parc des Expositions for €2.55 to CDG and €4.35 from) in order to supplement the Mobilis pass? Will this require exiting and re-entering the station to switch tickets (i.e. will the fare gates allow using a ticket for exit only)?