Migrate Devel FileCopy

This module contains a single migrate process plugin file_copy_or_generate. As the plugin's ID suggests, this migrate process plugin is able to generate files: if the specified source file is missing it tries to generate a file on the destination site.

How to use on behalf of Drupal core's file_copy process plugin

Implement the hook_migrate_process_info_alter() hook (undocumented afaik):

/**
 * Implements hook_migrate_process_info_alter().
 */
function [your_module]_migrate_process_info_alter(&$definitions) {
  // If the 'file_copy_or_generate' migrate process plugin is available (so:
  // Migrate Devel FileCopy is installed), then we replace the 'file_copy'
  // definition with 'file_copy_or_generate' – because we're probably debugging
  // a (properly sanitized) customer DB without having the source files.
  // @see https://drupal.org/project/migrate_devel_file_copy
  if (
    !empty($definitions['file_copy_or_generate']) &&
    !empty($definitions['file_copy'])
  ) {
    $definitions['file_copy'] = $definitions['file_copy_or_generate'];
  }
}