Migrate callbacks() with addFieldMapping for destination_dir
Maybe you can use file_class like this

$this-&gt;addFieldMapping('field_yourimage_field', 'fidsarray');
$this-&gt;addFieldMapping('field_yourimage_field:file_class')
-&gt;defaultValue('MigrateFileFid');


The you have to implement prepare row to get the managed file fids you want to import like this

public function prepareRow($current_row) {
 if (parent::prepareRow($current_row) === FALSE) {
  return FALSE;
 }
 //sample code to get the file ids. it may be different in your case
 $fidsarray= array(); 
 $fidsarray = explode(",", $current_row-&gt;columnwith_file_ids_comma separated);  
 $current_row-&gt;fidsarray=$fidsarray;
}


if you dont have managed files and therefore fids available other options can be used too.
See https://www.drupal.org/node/1540106