How to add a patch.c?

I try to build a package for NixOS. According to Archlinux package build code,, I need not only downloaded deb, but also a patch.c. However, src is not an array, I have

  src = fetchurl {
    url = "";
    sha256 = "sha256-HunwKELmwsjHnEiy6TIHT5whOo60I45eY/IEOFYv7Ls=";
    curlOptsList = ["-HUser-Agent: Wget/1.21.4"];

So How can I add a second src?

src is just a default name for the main source, so you can do

patch-src = fetchurl { ... }

and then add

postPatch = ''
  cp $patch-src patchme.c