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.
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