The following blog post by Jeremy Likness might be useful to provide you some insight about this:
Silverlight UI Automation Testing using Prism 4.0
Although it is mainly oriented to testing, it contains information that might be helpful for you to achieve your use case.
Additionally, the following article from MSDN might also be helpful for you:
UI Automation of a Silverlight Custom Control
In case the aforementioned articles aren't enough to clear your doubts, it would be helpful if you could provide us with more information about the scenario you're trying to achieve, and which are the problems that are blocking you.
I hope you find this helpful.
Guido Leandro Maliandi