Indeed there is no out-of-the-box way to do that with the current implementation of the RadImageEditor. However, you can get the ScrollViewer control from the RadImageEditor's template by using the FrameworkTemplate's FindName() method. If you want to read more about that topic, you can check this MSDN article.
For your convenience, I've also created a demo project which illustrates the above described approach. You can find it attached.
Hope this helps.
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