Well, it may not be as convenient as your suggested feature, but "Tools" -> "Select Elements" may already be used to select all nodes that are connected to the currently selected node (up to a certain graph distance). To do that, disable all options on tab "General", enable "Use These Criteria" for tab "Nodes" and said tab only, choose "Neighbors" for option "Select" and set the desired graph distance for "Maximal Path Distance" on tab "Nodes".