and not declaratively in the
configuration.nix like the rest of the system
Would it cause any problem doing that?
The main difference is probably that the channels are updated every time you rebuild. In most cases that is what i want. Maybe an option is needed to not update channels configured in configuration.nix when you just want to update configuration without updating packages.
Here are instructions how you configure unstable channel in
configuration.nix: how to add NixOS unstable channel declaratively in configuration.nix - Stack Overflow
How are you doing it?