An example of adding a new customer to Magento with a auto-generated password and custom welcome email.

add-new-customer-with-custom-welcome-email.php

<?php

// sending a custom password to customers

require_once('app/Mage.php');
umask(0);
Mage::app();

$store_id = Mage::app()->getStore()->getId();

$customer_email = 'some-customer@domain.com';
$customer_fname = 'John';
$customer_lname = 'Doe';
$passwordLength = 10;

$customer = Mage::getModel('customer/customer');
$customer->setWebsiteId(Mage::app()->getWebsite()->getId());
$customer->loadByEmail($customer_email);

if (!$customer->getId()) {

  $customer->setEmail($customer_email); 
  $customer->setFirstname($customer_fname);
  $customer->setLastname($customer_lname);

  $password = $customer->generatePassword($passwordLength);

  $customer->setPassword($password);

  try {

    $customer->save();
    $customer->setConfirmation(null);
    $customer->save(); 
    //$customer->sendNewAccountEmail();


    $templateId = 10;

    $sender = Array('name' => Mage::getStoreConfig('trans_email/ident_general/name'),
                    'email' => Mage::getStoreConfig('trans_email/ident_general/email'));

    $store = Mage::app()->getStore();

    $name = $customer_fname . ' ' . $customer_lname;

    $vars = Array('customer_name' => $name,
                  'new_password' => $password);
        
    $translate  = Mage::getSingleton('core/translate');

    Mage::getModel('core/email_template')->sendTransactional($templateId,
                                                             $sender,
                                                             $customer_email,
                                                             $name,
                                                             $vars,
                                                             $store->getId());
   
    $translate->setTranslateInline(true);


  } catch(Exception $e) {

    echo $e->getMessage();

  }

}