Trying berry window manager

I’ve installed berry ( nix-env -iA nixos.berry ). However no berry.desktop file is introduced to the system. Normally you would manually add berry.desktop to /usr/share/xsessions. Other window managers (bspwm, i3 etc) automatically add the desktop file in /nix/store/…

Is there a simple way of doing this?

Sorry - my mistake. All fixed.
I should have entered:
services.xserver.windowManager.berry.enable = true;
in Configuration.nix file