Paul, I don't think he's talking about proxies, since he already created a tutorial for them on pixelfondue:
https://www.youtube.com/watch?v=3eth8oXUN0IGreg, apart from using Octane proxies, there is no way to "import" an ORBX and have it automatically convert back to modo items/shaders.
You can use standalone to unpack the ORBX, then import all the alembic files into modo. You could also copy/paste the materials back into overrides (it's not perfect though).
This is obviously a lot of manual work