Hello,
Bulk-importing in one shot and auto-assigning to users can only be done with the Bulk Import add-on.
However, this add-on requires that you prepare, on your own (either manually or programmatically), a JSON file that contain all the data to be imported (such as post titles, owners, attachment files, etc…).
Do not hesitate to first take a look at the Bulk Import documentation.
Regards.
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