No, there is no workaround. When you already have an issued ticket, you can only get the agency that issued that ticket to make any changes to it.

It is always better to book the ticket you want first time, rather than changing your mind later and coming back to make changes to ticket you have already purchased. You can, of course, still make changes to your ticket - but as you're seeing it's a) not so straightforward, b) not so cheap, and c) more expensive than just buying the "new" ticket outright from the start would have been. (And this is not just Air France-specific; it applies to all airlines).

Of course, if the ticket you initially purchased was completely flexible and cancellable without penalty, then you would simply cancel the ticket for a full refund, and book the new ticket you want, and not need to get anyone else involved.

That's hardly surprising. You've asked to change a simple return into a multi-segment trip; that may not seem like much of a change to you, but in the eyes of any airline, that's a completely different product with its own pricing, to say nothing about becoming subject to new taxes (at the new stopover point) that would not have been levied if you were just a transit passenger. You also become subject to change fees (because, unlike a customer making a booking from scratch, you actually already hold a ticket and are bound by its terms and restrictions - which includes the levying of change fees when you decide you want to change the ticket).

If you book a stopover, it's technically no longer a return. So your terminology there is incorrect.

You can book anything you want. You can take a stopover wherever you want. However, beware that there may be (large) cost implications for this. This is because you are marking yourself out not as a passenger who wants to get from A to B as quickly and cheaply as possible - but you want to get from A to Paris, and then from Paris to B on some other date - so you are not one of the "get me there as quickly and cheaply as possible" brigade.

The problem for you, though, is that you already bought a (presumably restricted) return ticket. Now you've changed your mind. By already holding a (restricted) ticket, you are in a much worse starting position because you have "tied" yourself to a ticket with certain rules and restrictions.

In future, don't book your travel until you have decided what your itinerary actually needs to be...or, conversely, don't seek to change your itinerary in these kinds of ways (adding a stopover) once you have opted for a particular type of ticket (simple return).