There's likely a simpler way but the below seems to work.
cary = ["high schools" => {"Cary", "Green Hope", "Panther Creek"}, "population" => 116244,
"state" => "NC",
"weather" => "sunny"];
// 1.
cary << Insert("high schools", Insert(cary["high schools"], "another school"));
// 2.
cary << Insert("weather", Eval List(Insert(List(cary["weather"]), "humid")));
Show(cary);
/*:
cary = ["high schools" => {"Cary", "Green Hope", "Panther Creek", "another school"}, "population" => 116244, "state" => "NC", "weather" => {"sunny", "humid"}];