Hi Shane,
Thanks for submitting this suggestion.

If the lookup provider returns that a domain is not supported, then WHMCS will fall back to a standard WHOIS lookup automatically: https://github.com/WHMCS/sample-registrar-module/blob/master/modules/registrars/registrarmodule/registrarmodule.php#L903

So this should already be possible. Is there a particular lookup provider or search term for which this is not occurring?