Hello AM,
Generally RadChart does not support Box-and-Whisker series type but you can achieve similar effect through CandleStick re-templating (to add the additional Line visual(s)) and you will also need to create custom CandleStick / CandleStickSeriesDefinition pair of classes (to calculate the necessary coordinates for the additional Line visual(s))
See the attached sample application that demonstrates adding custom additional median to get you started.
Greetings,
Giuseppe
the Telerik team