Hello,
Unfortunately, it is not possible to assign an object using the user name.
However, it is possible to assign an object other than with the user ID. I invite you to read the Bulk-Import add-on documentation, and in particular the Mandatory properties section, where you can see the possible values. Some of these require the purchase of additional add-ons.
Please do not hesitate to contact us if you require further information.
Regards,
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