A lot of the functions you refer to rely on packages to provide the commands used to implement them, so the seperation between functions and packages (outside of lib at least) is not so clear.
If you want to have a custom package repository, you can either import nixpkgs to get access to the bits of it you want, or just maintain a custom Nixpkgs tree.
Personally, I prefer the latter, because it ensures that the version of Nixpkgs is always the version your custom packages were written for.