From looking into this s a few days ago it looked like things were packaged in the unstable channel so you can install them. But from what I read there is no way to configured it via nix configs so you would manually have to write them for now.
https://nixos.wiki/wiki/COSMIC
There is also this flake which should give some configuration for it but it mentions it might not be dully working yet and I have not yet actually tried it.
I don't expect things to change much in this regard though until they start releasing some more stable versions of cosmic which is still in a prealpha state.