Well, you can specify a tool tip text for each node that is displayed after a short delay if the mouse is over a node. To do so, select the node in question, go to the properties table in yEd's lower right corner, find the section "Data", and enter your additional information into the "Description" field.
If "Description" alone does not suffice, You can also create additional properties using "Edit" -> "Manage Custom Properties".
Finally, nodes and edges support multiple labels. To add a second, third, etc label, right-click the node/edge to open its context menu and choose "Add Label".
