There is a REST api available for NAbox, but it is not documented. It basically takes the same arguments as what you provide using the "nabox" cli once logged in as root.
You can also manually create configuration files in the /opt/conf/harvest directory. You must create one per system and respect the file naming convention so the interface is not confused.
Once you created the file, you can inject them in the main Harvest config file with the following command :
nabox harvest update-startup