The current Telerik Navigation framework is supported due to backward compatiblity with our previous releases when there was no Navigation framework in the Silverlight runtime.
If you are building a new project - it will be better to use the Silverlight framework from Microsoft.
More information about how to apply custom animation to RadNavigation you can find here:
Another approach could be to to use Navigating event which fires when navigating from one page to another or the Loaded event of RadPage.
I hope that this will help you.
the Telerik team