python: add existing level asset to persistent level via code

This seems like it should be super easy and probably is, but I seem to be missing something.

I found this post How do I make the Persistent Level become current after adding a sublevel with python? - Unreal Engine Forums which gives a bit of code to create a new sub-level. However when I run it, this code does not add/load an existing map asset, but overwrites my existing map with no data.

I have a series of maps that I need to add to a single “parent” level/map via code and trying to do this at the moment seems unnecessarily obtuse, or I am just doing something very wrong. I would imagine there would also be a “add_streaming_level” method(or something), but I have yet to find one anywhere in the python docs.

Any help is greatly appreciated