How to Add New Row to GridView without using datasource controls?

<div>
       <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="False"
           onrowdatabound="GridView1_RowDataBound">
           <Columns>
               <asp:TemplateField HeaderText="item code">
                   <ItemTemplate>
                       <asp:Label ID="lblID" runat="server" Text="<%# Bind('item_code') %>"></asp:Label>
                   </ItemTemplate>
               </asp:TemplateField>
               <asp:TemplateField HeaderText="Item name">
                   <ItemTemplate>
                       <asp:Label ID="lblItemName" runat="server" Text="<%# Bind('Item_name') %>"></asp:Label>
                   </ItemTemplate>
               </asp:TemplateField>
               <asp:TemplateField HeaderText="brand">
                   <ItemTemplate>
                       <asp:Label ID="Label2" runat="server" Text="<%# Bind('brand') %>"></asp:Label>
                   </ItemTemplate>
               </asp:TemplateField>
               <asp:TemplateField HeaderText="size">
                   <ItemTemplate>
                       <asp:Label ID="lblSize" runat="server" Text="<%# Bind('size') %>"></asp:Label>
                   </ItemTemplate>
               </asp:TemplateField>
           </Columns>
       </asp:GridView>
   </div>

protected void Page_Load(object sender, EventArgs e)
   {
       if (!IsPostBack)
       {
           var con = new SqlConnection(@"Data Source=.;Initial Catalog=test;Integrated Security=True");
           string statment = "select item_code, Item_name, brand, size  from Items ";
           SqlDataAdapter da = new SqlDataAdapter(statment, con);
           DataTable dt = new DataTable();
           da.Fill(dt);
           GridView1.DataSource = dt;
           GridView1.DataBind();
       }
   }
   protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
   {
       //Check if it's header
       if (e.Row.RowType == DataControlRowType.Header)
       {
                 //Create new Row in Gridview
         GridViewRow  row = new GridViewRow(-1, -1, DataControlRowType.DataRow, DataControlRowState.Insert);
             //create Cell and it will be added to row
          TableCell   cell = new TableCell();
           cell.Text = "0";
           row.Cells.Add(cell);


           TableCell cell1 = new TableCell();
           cell1.Text = "Item Name Test";
           row.Cells.Add(cell1);

          TableCell cell2 = new TableCell();
           cell2.Text = "eeeee";
           row.Cells.Add(cell2);

         TableCell  cell3 = new TableCell();
           cell3.Text = "ssss";
           row.Cells.Add(cell3);
                //add the row to Gridview and AddAT(Index of where it will be appear
            GridView1.Controls[0].Controls.AddAt(1, row);
       }
   }