There is no built-in support for importing Simulink models in yEd. However, according to the Simulink documentation, it is possible to export Simulink models to XML. Since yEd offers a general-purpose XML-based import you might be able to create XSLT stylesheets for importing Simulink XML into yEd.
yEd's XML import is available from "File" -> "Open" with "Files of Type" set to "XML + XSL (*.*)".