Currently the RadFilter allows customizing only this part of the FiledEditor where the user enters value to filter on. This can be replaced with a custom control of your choice as shown in the demo below:
You can use combobox, textbox, etc. The rest of the controls in the FieldEditor that server to choose the filter function and the property to filter on are part of the design structure of the control and cannot be replaced.
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed