The user migration classes support not only the core user properties like mail, name, etc., but also fields added by the core profile module, the Drupal 6 Content Profile module, or Drupal 7 custom fields. They take the following arguments:

• role_migration: The machine name of your role migration class, if any - used to set up dependencies and to properly map role assignments. If omitted, role assignments will not be migrated (they will default to authenticated user).
• picture_migration: The machine name of your picture migration class, if any. See below for a description of picture migration classes.

$user_arguments = $common_arguments + array(
  'machine_name' => 'ExampleUser',
  'description' => t('Import Drupal 6 users'),
  'role_migration' => 'ExampleRole',
  'picture_migration' => 'ExamplePicture',
);
Migration::registerMigration('DrupalUser6Migration', 
  $user_arguments['machine_name'], $user_arguments);