I can't find any way to add a new "subview" file to the theme. Is it possible? I already tried to export the theme, and reupload it with the new files, but they doesn't appear in the .twig listed files in theme editor, although I can see their effects.

Add it to the <template_editor> section of your config.xml file.

If you don't have that section, copy it from /themes/survey/vanilla/config.xml - github.com/LimeSurvey/LimeSurvey/blob/ma...illa/config.xml#L115

You may need to reset the theme to see changes.