ITransaction tx = session.BeginTransaction();

try
{
  AnObject obj = (AnObject)session.Get(typeof(AnObject), 1)
  obj.SomeProperty = "new value";

  SqlCommand sqlCmd = new SqlCommand();
  sqlCmd.Connection = session.Connection;
  sqlCmd.CommandText = "UPDATE MyTable SET MyField = "MyNewValue";

  tx.Enlist(sqlCmd);  //This is it...

  sqlCmd.ExecuteNonQuery();

  tx.Commit()
}
catch(Exception)
{
  tx.Rollback;
  throw;
}