What makes a flake config a flake config is simply the flake.nix entry point. So, technically if you read that file to see what file it loads for the nixosConfiguration you want to "port", you should be able to just go directly from that file and bypass the flake.nix.
For the longest time, my own flake simply forwarded to my configuration.nix.
However, depending on your needs of course, but using flakes even at a basic level can be very useful and I'd 100% encourage doing a basic setup for someone starting out. The main feature here is being able to lock your dependencies (including nixpkgs) to a specific commit, which means you will always get the same resulting setup (not depending on when you installed it, like it does without flakes). But, you know better than me the requirements of your own setup :)
Can confirm as well, awesome!!!