I’ve got some Nix expressions for the NetExtender client. I had to use some ugly hacks to get it working, including patching nixpkgs chrootenv.c. I never managed to get it to set routes properly, so I do that manually.
Here it is: https://github.com/spencerjanssen/dotfiles/tree/master/nixos/netextender
You’ll want to build default.nix then run the executable in result as root, that will put you in a shell with netExtender in the path. Happy to discuss any issues you might experience or improvements you find.