[DotNetDevelopment] how to add datagrd dropdownlist column to existing dataset table?

[Anju]

hi,
how to add new column to existing dataset table and fill that dataset
with dropdownlist selected values. any one pls help me.  when user
check the checkbox,droplist enabled for selecting the values.
dropdownlist values manually i added in datagrid.

<asp:TemplateColumn HeaderText="Duration">
                                                                                
                        <ItemTemplate>
                                                                                
                                <asp:DropDownList ID="ddlduration" 
Runat="server"
AutoPostBack="False" Enabled="False">                                           
                                                <asp:ListItem
Value="30" Selected="True"></asp:ListItem>
                                                                                
                                        <asp:ListItem Value="60"></asp:ListItem>
                                                                                
                                        <asp:ListItem Value="90"></asp:ListItem>
                                                                                
                                </asp:DropDownList>
                                                                                
                        </ItemTemplate>
                                                                                
                </asp:TemplateColumn>

private void btnSubmit_Click(object sender, System.EventArgs e)
                {
                        DataSet dscourse=(DataSet)ViewState["ds"];
                        DataSet selected = ((DataSet)ViewState["ds"]).Clone() ;

                        string total = lblTotalPrice.Text;
                        foreach (DataGridItem dgItem in dgExtendCourse.Items)
                        {
                                HtmlInputCheckBox chkclick = 
(HtmlInputCheckBox)dgItem.FindControl
("chkselect");
                                Label lblcource = 
(Label)dgItem.FindControl("lblCourseName");
                                Label 
lblDays_Left=(Label)dgItem.FindControl("lblDaysLeft");
                                DropDownList ddlDuration = 
(DropDownList)dgItem.FindControl
("ddlduration");

                                if (chkclick.Checked==true)
                                {


                                 foreach (DataRow row in 
dscourse.Tables[0].Rows)
                                        {
                                                if 
(row["coursename"].ToString() == lblcource.Text && row
["DaysLeft"].ToString()==lblDays_Left.Text)
                                                {
                                                        
selected.Tables[0].ImportRow(row);
                                                }
                                        }
                                }
                        }
                        if (selected.Tables != null && 
selected.Tables[0].Rows.Count > 0)
                        {
                                Session["selectedrow"] = selected;
                                Session["totalprice"] = 
HidenTotal.Value.ToString();

                            Response.Redirect("StudentRegistration.aspx");
                        }

                }