A small but convenient thing in both HM and NixOS module systems:
You can assign a package you're building like lib.mypkgs.pkg = pkgs.pkg.overrideAttrs()
Then you can reference it under
config.lib.mypkgs.pkg in the module system, or if you wanna nix build

nixosConfigurations.myhost.config.lib.mypkgs.pkg
homeConfigurations.myuser.config.lib.mypkgs.pkg

If you wanna be quick and dirty and can't be bothered to make options and don't need/want to overlay the package in nixpkgs.

Alternative: use an overlay and use your on sub attribute set:

self: super: { mypkgs.pkg = pkgs.pkg.override {}; }