Unfortunately, the ySVG extension package is not an extension for yEd (but for the yFiles for Java programming library) nor does it provide support for editing SVG documents.
You will have to either edit your SVG documents using a dedicated SVG editor application such as Inkscape or use a different approach for creating "corporate borders". E.g. you could use a simple rectangle node, style it to display the desired "corporate border" and title, add an additional label to the node and set the SVG document as the additional label's icon.