Hi @PabloOsorio 
You’ve rightly pointed out the documentation needs to be updated. It currently states:
To add secondary emails to a contact, when creating or editing the contact, in your request body, include emails in the hs_additional_emails field. Multiple emails can be separated by a semi-colon.
Given this property is “read only” (i.e. modificationMetadata.readOnlyValue = true), updating it via the relevant API V3 endpoints is not possible. As far as I know, the V1 endpoints are still operational, so I think you’ll have to use them for now.
I hope this proves helpful. Please let me know if you have any follow-up questions.