You can use SubtitleUrl for external caption file.
follow this code:-
function setVideo() as void
videoContent = createObject("RoSGNode", "ContentNode")
videoContent.url = "http://yt-dash-mse-test.commondatastorage.googleapis.com/media/motion-20120802-manifest.mpd"
videoContent.title = ""
videoContent.streamformat = "dash"
videoContent.SubtitleUrl = "pkg:/ttml/ttml.xml"
m.video = m.top.findNode("musicvideos")
m.video.content = videoContent
m.video.control = "play"
m.video.setFocus(true)
end function