Uploadcare indeed uses it's own CDN in case of CKEditor integration.
Most CDN providers will allow you to set base URL of content that you want to be available via CDN (e.g. http://www.example.com/uploads/images/). So that http://some-cdn.com/image.jpeg will distribute your image uploaded by ckeditor to http://www.example.com/uploads/images/image.jpeg
This is the easiest way to configure "PULL" CDN.