If named correctly, Plex will show a section of the file name as extra title.
TV Shows <- library folder
Show Name
Season XX
Show Name - sXXeYY - Optional Episode Title.ext
Show Name - sXXeYY - Optional Episode Title - Custom Extra Title-interview.ext
Show Name - sXXeYY.ext
Show Name - sXXeYY - Custom Extra Title-deleted.ext
The base name must be absolutely identical to that of your episode.
That being said… this will still limit what titles you can set. If the title contains a -, Plex will fallback to show the episode name instead.
There’s already an existing suggestion discussing an option to edit extra titles and other metadata from within Plex. Please comment/vote in that thread in order to help us avoid distracting or cannibalizing votes.
Unless I’m missing some unique aspect of your suggestion I’ll close this thread as a duplicate.