What do you mean by form data? What ever the data may be have it in datatable object. Then convert the datatable object to excel.
Use this code:
DataTable dataTable = new DataTable();
string attachment = "attachment; filename=Employee.xls";
Response.ClearContent();
Response.AddHeader("content-disposition", attachment);
Response.ContentType = "application/ms-excel";
StringWriter stw = new StringWriter();
HtmlTextWriter htextw = new HtmlTextWriter(stw);
GridView gvEmployee = new GridView();
gvEmployee.DataSource = dataTable;
gvEmployee.DataBind();
gvEmployee.RenderControl(htextw);
Response.Write(stw.ToString());
Response.End();