Assign all nintex tasks to another user

Example: User A has 10 tasks on different site(collections)  With the nintex farm task view the user can see that he has multiple tasks to do.

Is there a way an admin can see an overview of task of a specific users over the collections.  For 1 site it's easy to view just the list off course.

If user A leaves the company and does not delegate his tasks manually they keep assigned to A forever.

How can we (as admin) assign multiple tasks on different site collections to user B?