Not FCp. You can use DVD Studio Pro to do that properly. Not easy for sure.
In FCP, you can create subtitle by adding text, and then the description of the sounds to make it look like a closed caption. It will be ingrained into your image and I dont know why you'd want to do that...
You'd export a Quicktime Movie of your final edit and then open it in Magpie (developed by the nice folks at WGBH) and then either transcribe (OR cut and paste from your script) your captions and place markers for timing, and you'll have actual Closed Captioning (once you resave, of course) embedded.
This kind of text option is part of Quicktime. Someone just had to make an "editor" for it. There are others, some you can pay more for than you did for FCP.
I almost used this for some shows last winter but with the time crunch we were under and the amount of time to transcribe we used a service to do ours which then meant we ended up with a new BetaCamSP master that we had to then make a DVCam SubMaster of and then make appropriate dubs depending on what the stations would accept.
If you end up using this let me know how well it works for you.
I used to work for www.eegent.com writing closed captioning software. Their equipment isn't cheap as its more for post houses or networks. But if you are doing a lot of CC and you want real CC, not subtitles, they have everything you need.