Hi, at the moment (5.7.12) you can add 'Land Object' and buoy models within a world model, but not within a scenario.
So for example if you had a world model called TestWorld, then you could have
TestWorld
|- terrain.ini
|- etc...
|- Models
|- LandObject
|- MyLandObject1
|- MyLandObject2
|- Buoy
|- MyBuoy1
|- MyBuoy2
Separately, you can also add your own models of all types (Buoy, OwnShip, OtherShip, LandObject) and World models in your user folder (as shown if you click on the 'Open user folder' in the launcher). This means that even if you don't have write access to the installation location, you can add your own models. Within the user folder, you would have two folders, 'World' and 'Models', structured in the same way as in the installation.
So at the moment you can't wrap the custom models within a scenario folder, but I can see that this might be useful. I'll see how hard this would be to implement