You can install extensions with environment.systemPackages = with pkgs; [ gnomeExtensions.FOO ], but they won’t be enabled. You’d either have to enable them in GNOME Tweaks, or play with dconf. (I think home-manager has a dconf module that can make the setup declarative, but I haven’t tested it.)
Also, relevant (stale) nixpkgs PR: [WIP] nixos: add environment.gnome3.enableExtensions by bjornfor · Pull Request #46433 · NixOS/nixpkgs · GitHub.