The NixOS community has a lot of cargo culting going on, probably because it's not super accessible. Using Flakes and Home Manager for absolutely everything is a part of that in my opinion. I ended up going back and forth on both of them, my current opinion is that Home Manager makes sense (if you have a use case for it) but Flakes don't.

If you really need to import a flake-only project from a non-Flake config you can use builtins.getFlake. Personally if a project makes this required by inlining everything in a flake.nix I would consider that a sign of shoddy programming and it makes me reconsider using it, but it is possible.

If you're talking about using snippets of what other people have in their configs, then most of them should work perfectly fine. You may have to replace an inputs.whatever.nixosModules.default with a fetchTarball or an equivalent reference to an npins file but that's a pretty minor change.

For Home Manager such a translation doesn't exist because Home Manager does a lot of different things, and even when HM reimplements something that's already in NixOS (like HM dconf vs NixOS services.dconf) it usually doesn't quite act the same way, if only because most programs treat package, system and user configuration differently.