How to bulk update entities using the Feeds module, without existing mappings to a "unique target"
You could use a custom text field as unique target by using the "Unique values" validator of the Field validation module.

You could also implement hook_feeds_processor_targets_alter() in a custom module to add a "unique callback" to a target. In that callback you can implement your own logic for retrieving an existing entity.

See https://www.drupal.org/node/1307732#field_as_unique_id_7 for a list of steps for both solutions.
See also feeds.api.php for more information about implementing "unique" callbacks.