I've created a real simple app (time bombed beta) to convert your caption project to one with subtitles.
The app installs my TextUp Motion Title. This title will be used in the converted project.
Usage is simple: export XML, in the app set font and font size and drag the XML to the app. Alternatively drag the project from FCP browser to the app window.
The conversion is really very fast.
The changed XML will be saved to the desktop with a original project name plus " subs" name.
Once the XML is loaded in FCP you "batch set" things like line leading, outline, shadow etc.