<asp:SqlDataSource 
          ID="dsCategory" 
          Runat="Server" 
          SelectCommand="select CategoryID,CategoryName from Categories"
          ProviderName="System.Data.OleDb" 
	      ConnectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=С:\MAIL\main.mdb"> 
       </asp:SqlDataSource>        
       <asp:DropDownList 
          ID="cbRate" 
          Runat="Server" 
          AutoPostBack="True"
          DataSourceID="dsCategory"
          DataTextField="CategoryName" 
          DataValueField="CategoryID">
          <asp:ListItem Text="Все категории"  Value=99></asp:ListItem>  - хочу добавить здесь 
       </asp:DropDownList>                    

       <asp:DropDownList 
          ID="cbRate" 
          Runat="Server" 
          AutoPostBack="True"
          DataSourceID="dsCategory"
          DataTextField="CategoryName" 
          AppendDataBoundItems= "True"
          DataValueField="CategoryID">
          <asp:ListItem Text="Все группы"  Value=99></asp:ListItem>
       </asp:DropDownList>