IAddSubs - http://www.btobits.de/btobits/iAddSubs.html - play subtitles file while movie is playing in iTunes
iTunes Customer Service Contact - http://www.apple.com/support/itunes/contact.html - Apple states all sales are final, but you can always try.
I have purchased a TV series assuming that something as obvious as ENGLISH subtitles are included.
Lets say it still makes it okay because I only paid for the TV series, not the subtitles. I do not understand why itunes does not have a simple feature such as adding an srt file onto a movie.
Because Apple did not get the permission to allow this from the copyright holder (of the shows).
Since the files have DRM, changing them in anyway would break the DRM and make the file unusable.