At the moment it is not supported to add an animation to the RadPane when it is added.
What I could suggest is to add an animation for the whole PaneGroup
for each time the selected pane is changed - it will appear also when adding a pane (the new pane is selected when this happens) or when closing a pane (the currently selected pane is removed, so it is deselected). Unfortunately you cannot easily add animation only for opening/closing of a pane.
To achieve this you need to edit the ControlTemplate of the RadPaneGroup and to replace the ContentPresenter that presents the SelectedContent with a RadTransitionControl - it supports slick animations. For more information about how to use the TransitionControl into the ControlTemplate of a TabControl (the RadPaneGroup is actually a TabControl) you could refer to the following article - http://blogs.telerik.com/blogs/posts/10-05-12/using_transitions_with_radtransitioncontrol_and_transitionpresenter_in_radtabcontrol.aspx
Hope this helps!
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