WHMCS allow automated payments if one setup API, business or premier account i think.

But there is another idea how to go around that obstacle..:

Use CoinPayments.NET , they provide their WHMCS module and there one can accept cryptocurrencies and also PayPal. The money from customer are sent to any paypal address provided.

To do: create secondary coinpayments account, upload whmcs module, rename it so not ducplicate with existing one, in viewcart disallow paying if domain purchase.