Hey Euan,
first of all, thank you for the purchase!
The possibility to add outside links to the one-page layout menu has been added in theme’s version 1.5, following multiple requests from the theme users. You can now create a new menu in the “Appearance -> Menus” admin section, add the link(s) you need (only the external ones, because the one-page links are generated automatically based on the settings in the “One-page website” tab of the Theme Options panel), and assign the new menu to the “Main menu” location using the checkbox underneath.
Should you have problems adding the menu items, or have any other questions about Kanso, do not hesitate to contact me!
Sincerely,
Andrii / Satori Studio