Summary:
As per the current design, Web Resource does not allow placeholders to be imported instead of resources.

Note:
When specifying a resource to be assigned to a task, the resource must already exist in Web Resource for the assignment to be imported successfully. To determine which resource is being specified in the import file, the application will first attempt to find a User Name in Web Resource that matches the value. If a User Name matching the value specified does not exist, the application will attempt to match the value against the First Name or Last Name field in Web Resource. If multiple matches are found, the assignment will not be completed and a warning message will be logged.