Join the side of flakes
Just add a script in the same directory as configuration.nix that sets up your channels. This is one of various non-problems that flakes are supposed to solve.
You can also use fetchFromGithub if you want to pin to a particular version of nixpkgs, or anything else.
Providing some links on flakes:
The wiki is a solid starting point: https://nixos.wiki/wiki/Flakes
This video: https://youtu.be/S3VBi6kHw5c?si=oC0zXzQ8XWTCqM-y
Side note. Vimjoyer on YouTube (that video link above) is pretty solid. I like his content/tutorials a lot
You can pin a Nixpkgs commit in your configuration, without needing flakes
This is the first and only real reason I ever used flakes.
But you don’t need flakes, there are so many ways to solve this problem that don’t require experimental features.