You'll need to expose the $entity object first to see what you want to copy to where using the print_r($entity) or dsm($entity) first.
You want to copy the contents of the core node title into a CCK field with the machine readable name field_title.
Select the nodes you want to process with VBO as usual;
Select the VBO action "Execute arbitrary PHP script";
For Drupal 7, enter the following PHP snippet into the textarea:
$entity->field_title['value'] = $entity->title;
Make sure to replace field_title with the actual machine name of your field, which you can find on the CCK manage fields page.