Use this script to fill in the 'mail' field in the users table.

drupal7-add-email-address-to-users.php

<?php

$query = new EntityFieldQuery();
$query->entityCondition('entity_type', 'user', '=');
$query->propertyCondition('mail', NULL);
$query->propertyCondition('uid', 0, '>');

$query_result = $query->execute();

if (isset($query_result['user'])) {
  // EFQ query results are keyed by entity ID
  $user_ids = array_keys($query_result['user']);
  $users = user_load_multiple($user_ids);

  array_map(function($user) {
    $user->mail = $user->name . "@asu.edu";
    user_save($user);
  }, $users);
}