I was able to add a free stopover to my outbound and return flights. Original outbound, August 2021, was BUD->CDG->SFO on Air France with a 2.5 hour layover in CDG. Was able to add a 3 day stopover in CDG without any issues. Return flights are in August 2022. Original return flights in December 2021, which I cancelled, were SFO->DTW->AMS->BUD on Delta with layovers in DTW and AMS. Since it was a voluntary cancellation and with no "I" class being available on the requested travel dates on Delta, a nice ticketing specialist at Delta found me "I" class flights on Air France, SFO->CDG->BUD for August. A few weeks ago I found "I" class availability on Delta. Took a chance and called Delta to see if I could change my flights back to SFO->DTW->AMS->BUD. Explained to the ticketing specialist that at the time of rebooking there were no "I" class availability and I wanted to experience Delta One suites and if she could also add a 3 day stopover in AMS. Without any pushback she made the changes and added the stopover in AMS. From my own experience it is possible to add a stopover but YMMV.