<?php
function wpai_set_upload_folder_by_post_date($uploads, $articleData, $current_xml_node, $import_id) {
  if ( ! empty($articleData['post_date'])) {
    $uploads['path'] = $uploads['basedir'] . '/' . date("Y/m", strtotime($articleData['post_date']));
    $uploads['url'] = $uploads['baseurl'] . '/' . date("Y/m", strtotime($articleData['post_date']));

    if (!file_exists($uploads['path'])) {
      mkdir($uploads['path'], 0755, true);
    }
  }
  return $uploads;
}

add_filter('wp_all_import_images_uploads_dir', 'wpai_set_upload_folder_by_post_date', 10, 4);
?>

Plaats bovenstaande code in de function editor van WP all import.