There is a nix-home-manager module available in the repo as described in the relevant installation page.

Once you have installed it, you can set up automated syncing as follows:

programs.smos = {
  enable = true;
  sync = {
    enable = true;
    username = "YOURUSERNAMEHERE";
    password = "YOURPASSWORDHERE";
  };
};

A systemd user service will take care of the syncing for you. To protect your files against any potential bugs in the synchronisation, a backup service has also been provided. You can enable it as follows:

programs.smos = {
  enable = true;
  backup.enable = true;
};

Note that systemd timers do not run while a user is not logged in, so you may have to turn on lingering in order to automatically sync while the user is not logged in.

See also the reference documentation for this home manager module.