This is a functionality that is not yet present in the software, but you can check the following workaround:
You could assign the points of your stockpile to a new point group (say my_stockpile). You can then export this point group in .xyz format (only the my_stockpile point group). This should not be too large and you should be able to open this file in a spreadsheet application to compute max, min and avg. This will be for the altitude. For the height, you can subtract the altitude of the base plane.
Helpful articles to do this are:
Alternatively, you could export the volume object as a shapefile. You could then load the DSM and use this shapefile to define a region in a GIS software such as QGIS. You could then use the raster analysis capabilities of QGIS to obtain the desired statistics.
See herehow to export volumes.