After a bit more research it appears the recommended approach to take would be to simply make use of the Zammad API instead of creating a custom channel. I can easily make my own CRON job that pulls emails from the ZOHO API and imports them using the Zammad API.
The final problem I have is that I now run into the “No email address found for group ‘Users’ (1)” error since I can’t add an email account to #channels/email which means the ‘Users’ group Email field is blank.
I have my SMTP server configured fine for outbound email notifications so could I just hard code my email into the code preventing the error from appearing. Any idea if that change would have any knock on effects or indeed any ideas for a better solution?