Output mpeg-2 with 708 captions

Hi,

I want to create mpeg-2 videos with 608/708 captions. I drop a .mcc file into the Premiere Pro timeline. Then I want to export out mpeg-2 files for broadcast. I can create .mxf files with the captions. Why can't I create mpeg-2 files with captions? What settings am I missing. I have "embed captions" selected.

Thanks,

Jason