You would need to cancel your DOH-MEL and book a new CDG-MEL if available. Or buy a separate CDG-DOH hoping that flight changes won't hurt you.

On QR, like on most airlines, availability/cost for a married CDG-DOG-MEL is not the same (less) than on 2 segments.

Airlines cannot simply add a segment to an existing booking and then reprice it as if it was a single booking.