Kinda, you'd have to go through the whole review process and also get your stuff to successfully be part of (at least) an unstable build.
What really worries me is that on Nix we have a strong habit of patching packages to work around problems caused by the fact that NixOS doesn't use FHS. So, in theory, someone could submit an obscure patch to systemd and get everyone affected by it, if it through the approval process.
I'm sure PRs to the systemd package will get scrutinized by an experienced packager.
Also Nixpkgs isn't really that special when it comes to patching. Making thousands of software all work coherently on a single system inevitably requires heavy amount of pop patching. Nixpkgs actually handles this better by having easier ways to make customizations.
What really worries me is that on Nix we have a strong habit of patching packages to work around problems caused by the fact that NixOS doesn't use FHS. So, in theory, someone could submit an obscure patch to systemd and get everyone affected by it, if it through the approval process.
The solution so far has been a very rigorous process to even get a commit bit into nixpkgs: https://github.com/NixOS/nixpkgs-committers