You can add an SVG image to your diagram by simply dragging the SVG file in from the filesystem and dropping it in the diagram area. Dragging and dropping the URL to an SVG also work. The same idea works for PNGs, JPEGs, BMPs, etc.

Note that you cannot import an SVG representation of a diagram and edit it as a diagram, SVG does not contain the connector information needed to be able to determine which in the SVG are shapes and which are connectors.