Ok, so let me see if I understand this correctly.
So from what you've said I can load the save from Active GM Session and add new content like scenes, monsters etc...?
Yes, you can do that. Play mode allows to add content in the same way like in Prepare mode. So if you will load savegame without players, add scenes to it, save it again (if you don't plan players to join immediately) and next time you load freshly made savegame and players join your session they will have all the changes you did and their characters and equipment.
I can understand confusion you have about prepare mode and active session, but that separation exists because every active session is different in many ways depending on the choices players and gm do. So "Prepared campaign" should serve as barebone setup. Otherwise you will need to copy and adapt single campaign every time you want to start it with new players.
Imagine you have PnP book with the description of the campaign and basic scenario. They don't have every detail you need for the specific set of players because their actions can lead story in different directions. So prepare mode is the mode allowing to create such "books".