It can be done but it is not trivial. In essence you need to add a new block region and override the 'flat navigation' template and the code that supplies the context to add it. So there is https://moodle.org/mod/forum/discuss.php?d=231020 for adding a new block region to the code but that doesn't take into account templates which were not around at the time. So you'll also need to understand templates: https://docs.moodle.org/dev/Templates. Its also worth reading and understanding: https://docs.moodle.org/dev/Themes_overview.