Depends on the format you have them now.
If they are in a text generator inside the FCP timeline, use a tool from Spherico:
https://www.spherico.de/filmtools/TitleExchange/
If they are in another format, perhaps my subtitler can convert them for you.
https://www.videotoolshed.com/?page=products&pID=12
Also handy if you have them in text format, as it can import text quite fast. (Either import a complete file, or have it import selections while running in the background)
If you want to be cheap and have them in a text format, check the DVD SP manual on how to correctly format them. It aint rocket science, but you’re in for a treat if you have to do minor changes in DVD SP.
Bouke
http://www.videoToolShed.com
smart tools for video pro’s