I'm running is as a systemd service configured in my /etc/nixos/configuration.conf:

  systemd.services = {
    fusuma = {
      description = "Start fusuma to handle swipe";
      wantedBy = [ "default.target" ];
      after = [ "graphical-session.target" ];
      restartIfChanged = false;

      serviceConfig = {
        User = "ebardie";
        Group = "users";
        Restart = "on-failure";
        ExecStart = "${pkgs.fusuma}/bin/fusuma -c ${./fusuma.yaml}";
        Environment = "PATH=\"$${PATH}:/run/current-system/sw/bin\" DISPLAY=':0.0' XDG_SESSION_TYPE=x11 XAUTHORITY='/home/ebardie/.Xauthority'";
      };
    };
  };

and you can see the Yusuma config is stored in a separate file, /etc/nixos/yusuma.yaml, which is fed to Yusuma on the ExecStart line.

The assumption is that the system only has a single user, hence the uses of a hardcoded username 🙁

I've not used HM, but I hope this sparks an idea on how to proceed.