Oof... I've had a quick google search. But I am afraid, there is no "easy" to write that stuff to the actual metadata of the file.
If I recall correctly, Plex stores all it's data in a local SQLite database. So theoretically, you could query that database and then use some other tools to write those ratings to the database. So if you can code, you could probably whip something up in your preferred coding language and write that to the files.