You cannot add related entity's fields as "find columns" using OOB CRM. You can add columns from related entities as display columns in the view but not as find columns. Case and Email are related objects for a queue not directly stored in QueueItem
and that is why you are unable to add it in find columns list.
If this requirement is critical for you then you can develop a report or custom page that allows you to search on fields from related entities.
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