z9004158: 
You can see the current definition of 2bwm here: https://nixos.org/manual/nixpkgs/stable/#sec-stdenv-phases
If you want to patch the sources, you should probably add a `patchPhase` or `patches` attribute for this derivation. Read more about the different build phases here: https://nixos.org/manual/nixpkgs/stable/#sec-stdenv-phases
To add attributes to an existing derivation without having to clone and modify the nixpkgs repo see `overrideAttrs`: https://nixos.org/manual/nixpkgs/stable/#sec-pkg-overrideAttrs
And overrides: https://nixos.wiki/wiki/Overlays