I have posted a series of blogs on extensibility support in Coded UI Test.

1. Introduction to Coded UI Test Extensibility
2. “Hello, World” Extension for Coded UI Test
3. Introducing Sample Excel Extension
4. Technology Adapter of Sample Excel Extension
5. Property Provider of Sample Excel Extension
6. Action Filters of Sample Excel Extension

These should get anyone looking for writing extension for Coded UI Test, get started.  Use the Coded UI Test forum for questions\more info.

P.S. A related post: 7. Configure search properties used by recorder\code generation

More sidekicks:

8. Writing extension for WPF custom control

9. Extending supported properties set of a control

10. Writing your own client using UITest Object Model

More posts by Mathew Aniyan on this topic -

11. Custom WPF Control’s custom properties

12. Enabling UI Testing for Third party WinForms custom controls – 1

13. Enabling UI Testing for Third party WinForms custom controls – 2