It's easy to make changes in the theme xaml files. Since you are resourcedictionary merging these theme xaml files somewhere in your system, you just need to get the original file of Telerik.Windows.Controls.Input.xaml, make a copy, look for "NonEditableComboBox", and in "PART_DropDownButton", add:
And then point your ResourceDictionary Source to that xaml which you include in your project. There's help in the implicit themes documentation. Snoop is also your friend to find these elements in the xaml files.