Animate height of groupbox from 0 to auto

I have groupboxes acting like expanders in my application. When I need to colapse a groupbox I set its height equal to 0. when I need to expand it I set it's height equal to auto (double.Nan) is it posible to do this with a storyboard. How could I know the auto height in advance. Expression blend does not enable me to animate to an auto.