How to use field_attach_update for users
I don't know your function my_module_import_user return right data? I think user_save() work fine. You can try

if (isset($uid)) {
  $account = user_load($uid);
  $edit = array();
  $edit['field_FIELD_NAME'][LANGUAGE_NONE][0]['value'] = 'Field Value';
  // add more data
  user_save($account, $edit);
}


if you want use field_attach_update

if (isset($uid)) {
  $account = user_load($uid);
  $account-&gt;field_FIELD_NAME[LANGUAGE_NONE][0]['value'] = 'Field Value';
  // add more data
  field_attach_update('user', $account);
}


And my way, i often use Entity metadata wrappers

if (isset($uid)) {
  $account = user_load($uid);
  $wrapper = entity_metadata_wrapper('user', $account);
  //Set data of field
  $wrapper-&gt;field_FIELD_NAME-&gt;set('Field Value');
  // set more data here
  $wrapper-&gt;save();
}