Adding serialized data to users data column

The users table has a "data" column which can store serialized data, but you cannot migrate into this field by doing a mapping, because the user migration doesn't provide the "data" field as a possible mapping destination.

Instead, you can add the serialized data in your migration by implementing the prepare() method, which operates on the created entity:

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

(This information comes from cameronbprince at this issue. I have confirmed that it works.)