Issue while exporting dataset to excel

Hi,
I have a dataset and i want to export it to excel. Problem is while one of the datacolumn contains negative value and i want that column value to be shown in brackets in excel.

In the dataset (datatable view) its getting displayed correctly. i.e. (21,000) 

But in the excel, its ccoming as -21,000 but i want it in (21,000) format. Here is the piece of code which i am using...

*********************
   HttpResponse response = HttpContext.Current.Response;
  response.Clear();
  response.Charset = string.Empty;
  string filename = "test.xls";
  response.ContentType = "application/vnd.ms-excel";
  response.AddHeader(Constant.CONTENT_DISPOSITION, "attachment;filename=\"" + filename + "\"");
 
  using (StringWriter sw = new StringWriter())
  {
  using (HtmlTextWriter htw = new HtmlTextWriter(sw))
  {
  System.Web.UI.WebControls.DataGrid dg = new System.Web.UI.WebControls.DataGrid();
  dg.Font.Name = "verdana";
  dg.Font.Size = 10;
  dg.HeaderStyle.Font.Bold = true;
  dg.HeaderStyle.ForeColor = Color.SteelBlue;
  dg.DataSource = DATASET;
  dg.DataBind();
  dg.RenderControl(htw);
  response.Write(sw.ToString());
  response.End();
  }
  }



**********************

Please suggest something...
Thanks.