Thank you for your interest in RadWordsProcessing.
While FlowDocument inherits from ContentElement, this is not true of RadFlowDocument, so, unfortunately, you will not be able to host a UI element in it.
Please, keep in mind that RadWordsProcessing is a processing library which is entirely independent of any UI. In view of this I am not entirely sure why you would need to host a UI element in RadFlowDocument as you won't be able to show it and it will be lost upon export to the formats we support. If you'd like, you can tell us a little more about your scenario and we can discuss which part of it can be achieved with RadWordsProcessing.
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