Drupal 6 user password import to Drupal 7

I think you can create a page named something like rehash.php (in your root, same place as update.php). Then, log in as administrator first, browse to this page second. See code below (most taken from user_update_7200 in the latest drupal 7 install)...

Worse case, you could create a simple custom module and put this code in there.

Please note that you should back things up first:

<?php
    // bootstrap stuff
    define('DRUPAL_ROOT', getcwd());

    include_once DRUPAL_ROOT . '/includes/bootstrap.inc';
    drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);

    require_once DRUPAL_ROOT . '/' . variable_get('password_inc', 'includes/password.inc');

    // Lower than DRUPAL_HASH_COUNT to make the update run at a reasonable speed.
    $hash_count_log2 = 11;

    //  Hash again all current hashed passwords.
    $has_rows = FALSE;

    // Update this many users
    $count = 1000;

    $result = db_query_range("SELECT uid, pass FROM {users} WHERE uid > 1 ORDER BY uid", 0, $count);
    foreach ($result as $account) {
      $has_rows = TRUE;
      $new_hash = user_hash_password($account->pass, $hash_count_log2);
      if ($new_hash) {
        // Indicate an updated password.
        $new_hash  = 'U' . $new_hash;
        db_update('users')
          ->fields(array('pass' => $new_hash))
          ->condition('uid', $account->uid)
          ->execute();
      }
    }
?>