Yes, you can definitely add a quadratic trendline to your Excel chart. Here are the steps to do so:

1. First, create a chart in Excel with your data. Select the data range you want to plot and click on the "Insert" tab in the Excel ribbon.
2. Click on the chart type you want to create, such as a line chart or scatter plot.
3. Once your chart is created, right-click on one of the data points in the chart and select "Add Trendline" from the context menu.
4. In the "Format Trendline" pane that appears on the right side of the screen, select "Polynomial" as the trendline type.
5. In the "Order" field, enter "2" to specify that you want a quadratic trendline.
6. You can also customize the appearance of the trendline by selecting different line styles, colors, and thicknesses.
7. Click "Close" to apply the trendline to your chart.

That's it! You should now see a quadratic trendline plotted on your chart. If you want to see the equation for the trendline, you can check the "Display Equation on chart" option in the "Format Trendline" pane.