Hey Kjael,
this is not currently possible via the back-end, yet since you’re already the second person who is asking about this feature, I’ve decided to implement it and include into the next version of the theme (1.5). I’ve uploaded the updated files for you, please retrieve the archive here, unzip it, and upload the resulting file into the theme folder (/wp-content/themes/Kanso/), replacing the existing files. After that refresh the website – the menu items that scroll inside the page should remain, but now if you add a classic WP menu in the “Appearance -> Menu” admin section, they will also appear in the menu as normal links, to the right of the former. Please let me know if this works well.
Sincerely,
Andrii / Satori Studio