Is it possible to assign a lead owner during a list import?
No. The reason for this is because lead ownership is a Salesforce functionality and should be first done from within Salesforce. Here is one of the easiest options available to you within Marketo, if you are not able to do the assignment in Salesforce:
1) Upload yours leads into Marketo's Lead Database without lead owner information. i.e. your import CSV will not have any lead owner columns in it. More information on import leads at Import a List of Leads from a Spreadsheet into Marketo (Import List).
2) Once the list import is successfully completed and your leads are in Marketo, you can use the "Sync Person to SFDC" flow action to assign lead ownership. More information on the "Sync Person to SFDC" flow action at Sync Person to SFDC.