Edit the formatname field within CB > Field Management then under Display you should see a "Hover Canvas Content" parameter. This lets you customize the contents of the hover canvas. By default it just displays the online status and formatname field. You can completely override this to display whatever you like though using substitutions. See the below tutorial for substitution usage information.
www.joomlapolis.com/documentation/279-community-builder/tutorials/18353-using-substitutions-throughout-cb
It also has a trigger that can be used to extend it from CB Auto Actions. Specifically the onHoverCanvasDisplay trigger if you don't want to override the existing content. You'd simply use a Code action with Method set to HTML and under Output set it to "return". This would return your custom HTML and display it in the hover canvas.