How to add closed captioning to MPEG-2, QuickTime MOV, or AVI/DV for cable broadcast?
You could check out these links to see if they provide what you need:

http://www.theneitherworld.com/mcpoodle/SCC_TOOLS/DOCS/SCC_TOOLS.HTML

Caption Maker Pro:

http://www.cpcweb.com/

And of course Scenarist and Encore, although both are DVD oriented you should be able to produce at least MPEG-2 with closed captions embedded:

http://www.rovicorp.com/products/distribution/content-production-tools/scenarist-bd.htm
http://www.adobe.com/products/encore.html


  Closed-captioning support
  
  Import line 21 files to add closed captions to your DVDs (NTSC titles
  only).