It is not a supported feature of Pro Maps for google.
It cannot be used with the stand alone Pro Maps for google version which uses the V2 API, but the version in Design Extender uses the V3 API.
the documentation for adding the Drag n Zoom api can be found here:
examples.html
You would need to add the script reference for keydragzoom.js file to your page then edit the map js file created by pro maps in the webassist/google_maps folder and change:
map = new google.maps.Map(document.getElementById('wagmp_map_1'), myOptions);
to:
map = new google.maps.Map(document.getElementById('wagmp_map_1'), myOptions);
map.enableKeyDragZoom();
you would also need to add any event listeners you wanted to include by following the api documentation.