You can not add an optional subtitle track in Premiere Elements.
There are a number of programs which do allow you to create optional subtitles -- but which program we'd recommend depends on how much you want to spend and if you plan to output optional subtitles for YouTube, DVDs, BluRays, etc., and if you intend to work with Adobe software or if you don't care which brand of software you work with.
I have tools to create subtitles and store them as text files in SRT format, for example. I see commercial tools to take those subtitles and burn them as subtitle tracks. Some of those tools are plug-ins to Premiere.
What I prefer to find is a full life cycle tool that lets me import existing subtitle tracks, edit and maintain them, and then remux them back into the video. Really Premiere should be doing all of that, but since it doesn't is there a single third party product that does the whole life cycle development?
Once again, it depends on whether you require a professional tool -- like Encore -- or if you'd be happy with a consumer tool, like CyberLink PowerDirector.