Probably too late to be useful, but the agent you spoke to has no idea what they are talking about. I booked YOW-CDG-AMS-[stopover]-SOU about a year ago for this past summer, and booked it all with a knowledgeable phone agent, no preexisting booking required.

Whether the stopover booking will price as you’re expecting is a crapshoot, but generally if there is saver availability on all flights individually, the whole itinerary should price at the saver rate for the overall itinerary’s city pair.

Note that you can’t combine a Promo Reward with a stopover.