I think the first step is to implement your idea in CKEditor javascript code and then determine what you need to do. We do not have any way now to return selected text or the like back through the Vaadin adapter. That sounds like changes you need to make to CKEditor itself, but once you know how it’s done natively, there may be a way to allow that to work through Vaadin.
An earlier request for something that sounded simple, like getting the current cursor position, turned out to be complex in that most of the positions were relative to the nearest enclosing element, and not an offset from the HTML text buffer. If you can do it in javascript, though, then it can be analyzed to determine if it can be done via the Vaadin add-on. My guess is you will find you actually need to write a CKEditor plugin.