In reply to @grw00:matrix.org
anyway i think in the repo i refer to it's not so simple as colmena apply - error: Flake must define outputs.colmena

In reply to @schnecfk:ruhr-uni-bochum.de
Do you want to use a "shim" to populate outputs.colmena from outputs.nixosConfigurations?

iirc Buckley changed the approach for the shim, while I found the following solution.
In the flake.nix, I populate colmena from nixosConfigurations.

      colmena = {
        meta = {
          description = "My personal machines";
          nixpkgs = import nixpkgs {
            system = "x86_64-linux";
            overlays = [ nur.overlay ];
          };
        };

      } // builtins.mapAttrs (name: value: {
        nixpkgs.system = value.config.nixpkgs.system;
        imports = value._module.args.modules;
      }) (self.nixosConfigurations);

But to have the deployment options available (and not conflicting with colmena), I had to add:

        extraModules = [ colmena.nixosModules.deploymentOptions ];

in the nixpkgs.lib.nixosSystem call