there's an issue about php code to geneate joomla pass: using php to create a joomla user password? , any ideas how to get it done on the c#?

Your best choice is always to port the code Joomla uses to your own libraries as static techniques. All you want do ultimately is always to go ahead and take PHP process:

$salt = JUserHelper::genRandomPassword(32);
$crypt = JUserHelper::getCryptedPassword(password, $salt);
$dbPassword = $crypt.':'.$salt;

and convert it to C#:

string salt = Libraries.JUserHelper.genRandomPassword(32);
string crypt = Libraries.JUserHelper.getCryptedPassword(password, salt);
string dbPassword = string.Format("{0}:{1}", crypt, salt);