I would expect this to be the default PMS behavior. If the file name of the movie is exactly the same as that of the external subtitle file (except for the .srt or .ass ending, of course), it should automatically be picked up.
It is best to verify your settings.
Under "Transcoding Settings", click "MEncoder" and check the value for "Audio/subtitles language priority".
Make sure it contains something like "*,*" or "jpn,eng;eng,eng;*,eng;*,*".
The "*,*" allows the fallback to any audio language and any subtitle.
Also make sure "Automatically load *.srt/*.sub subtitles with the same file name" is checked.