Currently, there is an SDK example (https://github.com/telerik/xaml-sdk/tree/master/RichTextBox/WindowsMetafileFormatImageDecoder) showing how to implement a custom decoder that can import WMF and EMF images. We could integrate this decoder in the source code and enable the users to import their documents that contain such type of images out-of-the-box. 

Available in R1 2017 Official Release Version.