There's nothing stopping you for creating your own root CA and using it to sign certificates for any other domain. You can create a certificate for google.com if you wanted and be signed with your own CA.

Now, obviously, you couldn't actually use that certificate publicly. If you were to try to MitM someone, their client wouldn't accept the certificate because your root CA's certificate won't be in their trusted list.

But add that root CA to your own system, and it'll work fine.