When you orginally create the chart, include the series that will eventually be a line as a regular series in the stacked bar chart. Once the chart is created, right-click on the series that will be the line, select Chart Type, and then select line chart.
You might want to use a Stacked Column chart instead of a Stacked Bar chart though, since a column chart and a line chart can use the same axis labels, whereas added a line to a Stacked bar chart creates additional axis labels on the right and top of the chart, and the line won't go the same way as the bars.
-Ben
For this to work, the checkbox for "Apply to Selection" (under options) has to be checked. It took me a while to figure this out because it is not in an obvious place.