I have a trip coming up where I'll be flying from North America to Paris and onwards to China. The ticket is a business class seat booked in Z booked directly from Air France. I would like to make a stopover in Paris for a few days before heading over to Asia. The same flights are available at the same price for several weeks in a row.

How do you add a stopover to this trip? I looked at the fare rules and it states that stopovers are allowed for a nominal fee, but I can't seem to get it to show up on Air France's website using the multi stop search functionality. Is there a specific process to book a flight with a stopover or do you need to call in for this?