I misunderstood what you were after. To customize the button text, you have to create your own modal and instruct the extender to use it with the DisplayModalPopupID
property, like this:
<asp:Button ID="btnCancel" ruat="server" Text="Cancel With Confirm" />
<cc1:ConfirmButtonExtender ID="ConfirmButtonExtender" runat="server"
TargetControlID="btnCancel"
OnClientCancel="cancelClick"
DisplayModalPopupID="MyCustomModal" />
<cc1:ModalPopupExtender ID="MyCustomModal" runat="server"
TargetControlID="btnCancel"
PopupControlID="PNL"
OkControlID="ButtonOk"
CancelControlID="ButtonCancel"
BackgroundCssClass="modalBackground" />
<asp:Panel ID="PNL" runat="server">
Are you sure you want to cancel?
<br /><br />
<div class="buttons">
<asp:Button ID="ButtonOk" runat="server" Text="Yes (Custom!)" />
<asp:Button ID="ButtonCancel" runat="server" Text="No (Custom!)" />
</div>
</asp:Panel>