https://docs.whmcs.com/Changelog:WHMCS_V7.6
if you want to edit the template, you could always use {$smarty.now} to add a timestamp as the hostname... not random, but unless you get 2 users ordering at exactly the same time, it should work...
alternatively, you could use a ClientAreaPageCart hook, generate a random string, optionally check that it hasn't been previously used in the db, and then pass it back to the template as $server.hostname - but if you've already edited the template to add the other NA values, then you might as well just edit the template one more time! 😀