In the ticket 4510191 I logged to confirm this is missing I got this reply form Andy:
One quirk about importing Dynamic content is that it's an update, and not a create function. If you wish to add new dynamic content in bulk, I would recommend using our Dynamic content API: Create Item endpoint instead. ⬇
I understand that this is not the answer that you were looking for. My apologies for that! In this case, I will be marking this conversation as product feedback for review, which means that your input (the need for importing new Dynamic content through CSV import) will be aggregated as a part of our Voice of the Customer program that provides customer feedback to our product development teams.
So please speak up if you are a customer and need this functionality, leave a message here to encourage Zendesk product managers to add this.
The wierd thing is that upload functionality exists and works fine for adding translations in other languages but you have to create the Dynamic Content item with the default language one at a time manually.