Simply create a TCP/NNTP server that is bound to a local IPv6 network adapter for listening. For instance, using TIdNNTPServer, add an entry to its Bindings collection and set that entry's IPVersion property to Id_IPv6 and IP property to '::1' (or any specific local IPv6 address of your choosing), and then set the server to Active=true to begin listening.
Then you can connect TIdNNTP to that server using Host='localhost' and IPVersion=Id_IPv6.
Then you can connect TIdNNTP to that server using Host='localhost' and IPVersion=Id_IPv6.