Hi Haavard,
Thanks for taking the time to provide this feature request.

In the meantime, custom Merge Fields can be added to email templates to achieve this on your side.
First choose an API to get the whois data:
* Public Whois: https://developers.whmcs.com/api-reference/domainwhois/
* Registrar Whois: https://developers.whmcs.com/api-reference/domaingetwhoisinfo/

Then [EmailPreSend](https://developers.whmcs.com/hooks-reference/everything-else/#emailpresend) can be used to create your own smarty variables to use on email templates.