I don't believe you can do it from gen media already created.
You REALLY want to rethink your workflow. The third-party product Vegasaur in combination with other resources will become your best friend. Create titles, outside of Vegas with text editor, use Vegasaur to import and create gen media on the fly, and be able to simultaneously export the correct format for DVDA subtitles.
<<Create titles, outside of Vegas with text editor, use Vegasaur to import and create gen media on the fly, and be able to simultaneously export the correct format for DVDA subtitles. >>
Thanks for the tip, I was struggling with this as well.