You might want to look into the beta for the new/future Plex tv-show scanner/agent which provides native support for local extras (not just mistreated specials).
Extras can be added similarly as for movies on show/season levels. Episode extras are a bit more tricky for now.
Example:
TV Shows <- the folder linked to your tv-show library
Show Name
Season XX
Show Name - sXXeYY.ext
Show Name - sXXeYY-deleted1.ext <- episode extra (numbered)
Show Name - sXXeYY-deleted2.ext <- episode extra (numbered)
Show Name - sXXeYY-behind.ext <- single episode extra
Behind the Scenes <- season XX extras folder
Gag Reel.ext
Trailers <- show level extras folder
Show Trailer.ext
Clean Opening Titles-other.ext <- other show level local extra (inline)