Is this possible to do? I’ve looked at using EditorLevelUtils.create_new_streaming_level, but this appears to create a “Transient” level that doesn’t remain when the persistent level has been saved and reopened, and it doesn’t seem possible to create multiple sublevels with it (it will just remove the existing sublevel when a new one is created). Other then that I haven’t found anything that seems to allow either creating a new sublevel, or adding an existing level as a sublevel.
In case you haven’t found anything, this post seems to cover creating sublevels in Python: How do I make the Persistent Level become current after adding a sublevel with python? - Editor Scripting - Unreal Engine Forums