// Термины
class TermMigration extends Migration {
  public function __construct() {
    ...
    $this->addFieldMapping('tid', 'some_primary_key');
  }
  public function prepare($term, $row) {
    db_query("INSERT IGNORE INTO {taxonomy_term_data} (tid) VALUES (?)", array($term->tid));
  }
}
 
// Комментарии
class CommentMigration extends Migration {
  public function __construct() {
    ...
    $this->addFieldMapping('cid', 'some_primary_key');
  }
  public function prepare($comment, $row) {
    db_query("INSERT IGNORE INTO {comment} (cid) VALUES (?)", array($comment->cid));
  }
}

Drupal 7, Migrate 2.5