Could you please clarify whether you want to create and add a custom dropdown to the toolbar or you have already created a custom dropdown and you want to find it?
If you want to create a custom dropdown see this live demo: Custom Dropdowns
If you want to find a tool on the toolbar use the RadEditor1.FindTool
(name) server method of RadEditor. The RadEditor1.EnsureToolsFileLoaded(); should be fired as well before searching for any tools on the toolbar.
the Telerik team