[qazxcvbnm]

Pinning in nix is rather easy; either `nix registry pin nixpkgs` or explicitly `nix registry add nixpkgs github:NixOS/nixpkgs/925b70cd964ceaedee26fde9b19cc4c4f081196a` does the job. For specific packages, flakes and flake locks manage this fairly automatically as well. Do they not suffice for you?

For grabbing a package from another version of nixpkgs, its quite straightforward to only use that specific nixpkgs for one package and nothing else. NixOS also keeps old generations around in case the new configurations break anything, so I'm not very worried about the OS being rendered unbootable in any case (although I've never managed to break my NixOS, so I can't attest to having made use of this personally).