You can use the tool from menu Analysis:Mathematics:Average Multiple Curves... But you may need to first extract the data of the distribution curves before running the tool. See the attached video:
Another options is go back to worksheet with all columns highlighted and choose Statistics: Statistics on columns. On Input tab, set Input Data to be Combined as Single Dataset. On Plots tab, check Histogram.
U can double click the histogram to show distribution curve and not show bins in Plot Details dialog.