The way I grok the situation here -
To add a row (such as a naming_series field) to a DocType is a server side customization.
Such a change requires console access for eg ‘bench set-config developer_mode 1’. See https://frappe.io/docs/user/en/guides/app-development/how-enable-developer-mode-in-frappe
erpnext .com cloud supports custom app installs (limited to ‘large’ accounts), but I am not sure this host service includes console access, hence the question of server side customization support. I could be mistaken here and so seek informed sources.
Given that distinction (ie server versus client side customization) I thought best to start a new thread for a response to your question and an update.
Maybe please reiterate or clarify what is it you wish to do that you cannot and/or seek an update on?