export an XML from FCP
go to https://www.michaelcinquin.com/tools/subtitles/list_subtitles
upload xml and export as stl (dvd studio format)
then open jubler (you can download the app here:https://www.jubler.org/download/index.php)
open the stl file
choose “save as”
choose subrip (srt) in format and save
DONE !