@otini Yeah, same thing here (but for different packages) 😟
You should probably use the following expression instead of plain xmonad (untested): xmonad.override = { packages = [package1 package2]; }
If you look into nixpkgs/pkgs/applications/window-managers/xmonad/wrapper.nix, you'll see that the function it defines takes "packages" as an argument and uses it as a list of packages to install GHC with. That's where I took the "packages" attribute from. I hope this helps (because if is doesn't I have no idea what to do 😃 )
@otini Arch pro < NixOS noob