{
  "zlib" = callPackage
    ({ mkDerivation, base, bytestring, QuickCheck, tasty
     , tasty-quickcheck, zlib
     }:
     mkDerivation {
       pname = "zlib";
       version = "0.6.3.0";
       sha256 = "1nh4xsm3kgsg76jmkcphvy7hhslg9hx1s75mpsskhi2ksjd9ialy";
       revision = "5";
       editedCabalFile = "0mj3f7ql54p0pfa5a6q4xv4ci8xf00616ls0nyadpmlqbb4qilwn";
       libraryHaskellDepends = [ base bytestring ];
       librarySystemDepends = [ zlib ];
       testHaskellDepends = [
         base bytestring QuickCheck tasty tasty-quickcheck
       ];
       description = "Compression and decompression in the gzip and zlib formats";
       license = lib.licenses.bsd3;
     }) {inherit (pkgs) zlib;};
}

this is from hackage-pacakges.nix, and I'm not sure if I have any chance to override zlib when callPackage is already called.

So you want to be able to e.g. add extra arguments to the cabal2nix command?

IIRC that's possible with callCabal2nix and obviously if you implement the command yourself. Overriding is trickier but should still be possible through overriding callCabal2nix (this isn't pretty).

I'm having trouble understanding why overriding the result is insufficient in your case.