Zoho Books -> CRM Custom field sync for contacts

Zoho Books -> CRM Custom field sync for contacts

I use a custom field for my three different brands. Since I wanted to start a newsletter I recogniced there is no option to import from Zoho books to Zoho campaign. Only through CRM. I can't sort my customers because the custom field is not synced to crm.

What's the best workaround to start newsletters with zoho books?
I dont really use crm tbh.