Telerik RadControls for ASP.Net AJAX supports third party controls. RadControls such as RadMenu, RadComboBox etc support embedding third party controls in the Template. Please have a look at the sample provided for RadMenu.
ASPX:
<telerik:RadMenu ID="RadMenu1" runat="server">
<ItemTemplate>
... //embed third party controls here. </ItemTemplate>
</telerik:RadMenu>