Such customization option is not available with the CropTool. However, you could implement a custom tool that will fit this scenario. For your reference, I've extracted the code of the default tool in a demo project showing how you can go about the task of creating a custom tool. Note that you'd need to change the way the adorner looks and behaves and the Execute method of the command as right now it returns a bitmap with a rectangular shape.
I hope this is helpful.
Regards,
Petya
Telerik
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Feedback Portal and vote to affect the priority of the items