when I did a very similar thing in the post referenced by pimiddy, I used a custom nixpkgs github repo to create an overlay of nixpkgs. Check out this post where dropol explains how to do it. (in the end I took a slightly different approach that didn’t use flake-parts). Then, using the flakes referenced in pimiddy’s post, I was able to import my custom python packages into any flake I want, by using my custom nixpkgs as an input to the flake.