I created a plugin for this feature, see http://www.highcharts.com/plugin-registry/single/5/Rounded-Corners
Edit 2014-05-07: Co-worker Stephane also has a nice drop-in code snippet for creating custom shapes: http://jsfiddle.net/highcharts/My9Qp/Andreu Araque commented
Right now, when the serie on top has a 0 value there is no border radius in that column and it looks strange. There are 2 options, add this in the plugin itself or be able to add a border radius in a data item instead of the complete serie. What do you think?Andreu Araque supported this idea ·Andreu Araque commented
I think this is an important issue. It is necessary for stacked column charts.