Adding resource strings to language file?

I would like to add some additional text strings to the file [wwwroot]/html5viewer/resources/locales/mapping.en-US.json.js, but when I add them to this file they are still unavailable when assigning the label string in Essentials Manager.  Are these strings compiled in to some other file?  I assume it must be possible but maybe I'm wrong?

I've tried searching the forums and read the Essentials and HTML5 admin guides but I don't see anything that relates to adding/using your own custom text strings.  Does anyone have a good how-to on doing this?

Peter.