why importer cannot map to custom field type
You may also need to specify the field mapping details in:

hook_feeds_processor_targets_alter


  Alter mapping targets for entities. Use this hook to add additional target options to the mapping form of Node processors.


Example implementations: http://www.drupalcontrib.org/api/drupal/contributions%21feeds%21feeds.api.php/function/implementations/hook_feeds_processor_targets_alter/7

More info: http://www.drupalcontrib.org/api/drupal/contributions%21feeds%21feeds.api.php/group/mappingapi/7