It is possible to export video  with burned-in subtitles with Limecraft. Currently this feature is in private Beta and you can read more about it in this article

On many platforms such as Facebook, Vimeo, Linkedin and Youtube, you can import sidecar subtitle file, which gives you the option of switching the captions on and off. However, burning in subtitles allows uploading your content on a variety of platforms, including ones that do not support sidecar captions as well as control over the look and design of your subtitles.