I've read a lot, but didn't get a simple answer for 3 topics I'm interested about:

1. Are there any 3rd party repos for NixOS? It seems some people overlay one package, but I cannot find entire packagesets such as the one in Nixpkgs. Are channels used for that? Would I be able to have my own repo with several packages? Can I have more than 1 repo in my Nix system, and set which package install each program from?
2. Would it be possible, for propietary software compiled directly for NixOS, to have a binary-only (as opposed to source, or binary+patched) repository? Everything I see is either source+cache or binary+patched, but nothing like binary especifically for NixOS.
3. Let's say I want to add some extra, propietary codecs unavailable to NixOS. How would I be able to change a dependency so that each package dependent on it automatically picked it? Could I switch a dependency for a certain repo from one of another, different repo?

Thank you for your support guys!