I don't know if a mechical/automatic process is possible, but it works for haskell and python packages from Hackage and PyPI.

nix packages are not exactly packages, as in rpm or deb files, they are expressions in a functional language (think build recipes). there is a central repository of recipes on github: https://github.com/NixOS/nixpkgs/tree/master/pkgs

but you can make your own, and eventually make a pull request. They are more similar to Arch's PKGBUILDs. Packages in the central repository are constantly built by a continuos built process (http://nixos.org/hydra/), that works both as a binary cache and as a testing system. This is an expression I wrote to install SuperCollider plugins https://gist.github.com/gosub/a42e265ec38d9df203d6 using the Arch PKGBUILD as a reference https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=sc3-p...