Adobe's docs describe how to apply an outline to the pie chart by using the mx:stroke element within a PieSeries, however this only changes the border of the entire chart.
How do I add a border around each of the wedges in the chart?
There's two properties:
stroke
(the stroke around the circumference)
and
radialStroke
(the stroke within the pie chart itself)
If you're successfully setting one then the other is applied in the same manner.
Josh