While reading questions of how to make/use Closed Captions as Open Captions (Burned in) in many groups and forums, here a short answer.
Within FCPX convert the CEA stream to ITT and export as ITT.
Open the ITT with X-Title Importer (version should be newer than 3 or 2 years) and convert to fcpxml with Motion Templates of your choice.
Depending on the age of your X-Title version you may need to change the .itt extension to .xml
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