Adding serialized data to the users.data field

In case someone else is looking...

The default Drupal users table has a column called data which stores a serialized array of fields and values. This data field isn't included with the normal fields provided by MigrateDestinationUser::getKeySchema() and I wasn't able to find any documentation for migrate that demonstrated how to populate this field. I dug around in the code and after a little trial and error came up with the following function which should be added in your user migration class:

public function prepare(stdClass $account, stdClass $row) {
  $account->data = array('field1' => 1, 'field2' => 2);
}