/**
 * Implements hook_migrate_prepare_row().
 */
function {{ machine_name }}_migrate_prepare_row(Row $row, MigrateSourceInterface $source, MigrationInterface $migration) {
  if ($migration->id() == 'd6_filter_formats') {
    $value = $source->getDatabase()->query('SELECT [value] FROM {variable} WHERE [name] = :name', [':name' => 'mymodule_filter_foo_' . $row->getSourceProperty('format')])->fetchField();
    if ($value) {
      $row->setSourceProperty('settings:mymodule:foo', unserialize($value));
    }
  }
}