Adding a subtitle to a chart title in Excel is a great way to provide additional information about the data being presented. Here's how you can do it:

1. First, select the chart you want to add a subtitle to by clicking on it.
2. Next, click on the "Chart Title" option in the "Chart Elements" section of the "Chart Design" tab in the ribbon.
3. Click on the "More Options" button (represented by three dots) that appears next to the chart title.
4. In the "Format Chart Title" pane that appears on the right side of the screen, click on the "Text Options" tab.
5. Under "Text Box," click on the "Text Box" dropdown menu and select "Custom."
6. In the "Custom" section, you can now enter your subtitle in the "Text" box.
7. You can also adjust the font, size, color, and alignment of the subtitle using the options provided in the "Text Options" tab.
8. Once you're happy with your subtitle, click "Close" to apply the changes.

Your subtitle should now appear centered below the chart title on your pie chart.