There is no access to indicator levels from API, but you can add on chart price marker instead:
IChart chart = context.getChart(Instrument.EURUSD);
IIndicator adx = indicators.getIndicator("ADX");
adx.getOutputParameterInfo(0).setLineWidth(2);
IChartPanel adxPanel = chart.add(adx, new Object[] {14});
IChartObjectFactory factory = chart.getChartObjectFactory();
IPriceMarkerChartObject marker = factory.createPriceMarker();
marker.setPrice(0, 20.0);
marker.setLineStyle(LineStyle.DASH);
marker.setDecoration(IDecoratedChartObject.Placement.Beginning, IDecoratedChartObject.Decoration.None);
marker.setDecoration(IDecoratedChartObject.Placement.End, IDecoratedChartObject.Decoration.None);
adxPanel.add(marker);