Build it as multi-city at CO.com or a third party and see what happens.

Generally speaking, if you have a connection and want to make it a stopover, it adds to the cost, since most lower fares don't allow for free stopovers.

That said, I don't see any fares published on CO from EWR-JTR, so it's possible it's calculating it like you're paying for EWR-CDG + CDG-JTR separately anyway. If that's the case, it shouldn't matter if you spend more time in CDG, so long as the same availability is there on the flights you pick.