How to get KDE Plasma 6?

Hey guys, I’m pretty new to Nix and NixOS and one of the reasons I installed it is because I know there’s some way to install the KDE 6 Beta before the official stable release. I wanted to use the kde2nix overlay, but appearantely KDE 6 has been merged into the official nixpkgs repository. How do I switch from Plasma 5 to 6?...