I know you can if you place the chart on it's own ChartSheet.
Right-click on the chart and choose Location, then you can move the chart to it's own sheet and place a footer via the usual File/Page Setup/Header-Footer's method.
If it were me though, more likely I'd fudge in an extra Legend key or make an X-Axis title with the footer text I want (via Right-Click/Chart Options).
HTH