U can extendextend welcome functionality Email and ( Mage_Customer_AccountController ) with and send own random generate password to user and save in customer account.
i have manage before 2 days see hear but my condition in small change.
$customer = Mage::getModel("customer/customer");
$customer->setWebsiteId(Mage::app()->getWebsite()->getId());
$customer->loadByEmail($login['username'])
$customer->setPassword($customer->generatePassword(8));
$customer->save();
$customer->sendNewAccountEmail();