If your intended end product is DVD, then you should use your subtitle file in either DVDStudioPro or Encore. For DVDSP the STL-format is preferred, ENCORE requires the Adobe.txt format as a UTF-8.
Programs that can help you get the right format for your needs are BELLE NUIT or TITLE EXCHANGE. The latter can actually convert subtitles created in FinalCut (not recommended) into STL and other formats. BELLE NUIT is also very good. If you have access to Windows there are freeware programs that help convert subtitle formats (e.g. Subtitle Workshop)
If you really need the subtitles in FCP then you must render the subtitles as subpictures (graphic) and also the timecode as a separate file (edl or xml).
To add your subtitle file to your dvd-project, you import the Subtitle-file in proper format as a script to the respective Subtitle track in the timeline (in DVDSP or ENCORE). For DVDSP the font specified in the subtitle file must be installed on your machine.
Good luck,
ninetto