Unable to export internal subtitles in CEA-608 format

Hi, I opened an mp4 file that contains internal subtitles in the CEA-608 format(?), but I cannot export those into an .srt file (the Export command is greyed out)... I don't know if this is an error, a missing feature, or a for some reason unsolvable task, but it would be great to know if there is any work around in order to be able to get these subtitles into an editable text file -- thanks, Mark