The problem with override is that it depends on initially computing the derivation successfully. This doesn't work if an argument is missing which is the case here with testu01. That's why callCabal2nix has a third argument to pass in arguments (which can also act as an override at the definition site). So you want to do:

(pkgs.haskellPackages.callCabal2nix "splitmix" (builtins.fetchTarball "https://hackage.haskell.org/package/splitmix-0.1.3.1.tar.gz") { testu01 = null /* just pass something */; }).override { testu01 = /* later we can pass something else: */ my-pkg; }