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.
As I'm the author of TitleExchange I may chime in.
TitleExchange can convert a lot of common subtitle formats into generators of your choice within a Final Cut Pro sequence (or DVD Studio, or Avid, or DCP or ...). Depending on the in/out format a lot of subtitle settings can be transported.
To create subtitles, SubBits or BelleNuit are recommended. TitleExchange can work with both of them to create matching generators of your choice within Final Cut Pro. Different from those tools TitleExchange does not create bitmap graphics or does spotting. That maybe bad in some cases but could help in others as you are way more flexible.
If you render the subtitles using SubBits or BelleNuit you don't need the font installed on your target machine. But you can't make any changes to the titles directly within your FCS application.
Just as a note: I don't think Final Cut Pro (and the generators available - except Boris) are that bad for subtitles as long as you know what you're doing. I know quite a lot of theatrical releases where subtitles where done with Final Cut Pro - not the spotting of the subtitles.