How to import in $account->data field?

Hello,

After some time I decided to ask for little support on this. I must be dumb :(
My problem is how to migrate some data from custom table's field (longtext) to users data. I tried following code, no success:

function mymodule_migrate_fields_user($type) { 
  $fields = array();
  $user_fields['data'] = t('User: Data');
  $fields = array_merge($fields, $user_fields);
  return $fields;
 }

function mymodule_migrate_prepare_user(&$account, $tblinfo, $row) {
  foreach ($tblinfo->fields as $destfield => $values) {
    if ($values['srcfield'] && isset($row->$values['srcfield'])) {
      $newvalue = $row->$values['srcfield'];
    }
    else {
      $newvalue = $values['default_value'];
    }
    if ($destfield == 'data') {
      $account['data'] = serialize($newvalue);
    }
  }
  return $errors;
}

It simply won't go ... Any tip much appreciated. Thanks!