Carrie,
If you do not already have one, make a column that sums your stacked data. You can add this series to your existing stacked column graph on a secondary axis, make your trendline, and then format the summary data invisible (no border, no area).
How:
Right click on your graph.
Select Series tab.
Click on Add button
For Values select the column of summed data
OK
Now the summed data series sits stacked on top of your other stacked data.
Right click this added series on your graph.
Format data series
Select Axis tab
Click Secondary axis radio button
OK
Now the summed data series sits on top of, and is hiding, your stacked data.
Right click on this series (again).
Select Add trend line
Chose the type you want
OK
Your stacked data is still hidden on the graph, but now you have a trendline for the summed data.
Right click on series (yet again!).
Select Format Data Series
Select Patterns tab
Click None for both Border and Area radio buttons
If you have a legend you will have to select the summed data series and delete it, and you should be good to go.