Hey RGB,
submenus are only possible in the multi-page mode in Kanso; this has been done intentionally, to keep the navigation as simple and usable as possible – some users become confused when both scrollable one-page menu and submenu co-exist :) I must apologize for not making it clear enough in the manual, will correct this. I would recommend trying to add the extra items as top-level items into the current one-page menu; in principle I could help you add a bit of the native WP menu to the existing scrolling menu, yet this will work only if the item with the submenu is either the first or the last one in the menu. If that is the case, please let me know, I will provide instructions.
Regarding your second question: I’ve decided to give it a shot. Please download the custom files from my repository, unzip the archive, and upload the contained files into the theme folder (/wp-content/themes/Kanso), replacing the existing files. Let me know if it works!
Sincerely,
Andrii / Satori Studio