gistfile1.js

function WidgetConfig() {}
WidgetConfig.prototype.__fields__ = [];
WidgetConfig.addFields = function (newFields) {
  var Super = this;
  function Child() {
    return Super.apply(this, arguments);
  }
  _.extend(Child, Super);
  Child.prototype = Object.create(Super.prototype);
  Child.prototype.__fields__ = Super.prototype.__fields__.concat(newFields)

  return Child;
}

Just start a new inheritance chain.

var CustomWidgetConfig = WidgetConfig
.addFields([FORMAT_FIELD, DATA_FEED_URL]);
var PollingCustomWidgetConfig = CustomWidgetConfig
.addFields(POLLING_FIELDS);

var HighchartWidgetConfig = WidgetConfig
.addFields([HIGHCHART_DATA_FEED_URL]);
var PollingHighchartWidgetConfig = HighChartWidgetConfig
.addFields(POLLING_FIELDS)

// Etc...