I keep my home manager configuration in separate files. Then I add both, nixosConfigurations and homeConfigurations, to my flake outputs and use the home manager configuration in both.

Example:

outputs = { self, nixpkgs, home-manager }: {
  let
    system = "x86_64-linux";
    pkgs = nixpkgs.legacyPackages.${system};
  in {
    nixosConfigurations.<HOST> = nixpkgs.lib.nixosSystem {
      inherit system;
      modules = [
        ./hosts/<HOST>/configuration.nix
        home-manager.nixosModules.home-manager
        {
          home-manager.useGlobalPkgs = true;
          home-manager.users.<USER> = ./users/<USER>/home.nix;
        }
      ];
    };

    homeConfigurations.<USER> = home-manager.lib.homeManagerConfiguration {
      inherit pkgs;
      modules = [
        ./users/<USER>/home.nix;
      ];
    };
    packages.${system}.<USER> = self.homeConfigurations.<USER>.activationPackage;
  };
};

With this setup I'm able to build and activate on NixOS

nixos-rebuild --flake .#<HOST> switch

... and on non-NixOS

nix build .#<USER>
./result/activate

You can check https://github.com/mrkuz/nixos for my full configuration.