It is possible but it would cause the standard parcel shelf to bow slightly due to the weight.
Go halfrauds and go to the Ripoff desk. Ask them to order you an acustic parcel shelf with the holes pre cut for whatever size speakers your going to use. Will cost you £110 for the shelf and then you just fix your speakers in the holes on the underside
Btw. I know it will work as I've already done it to an Arosa and a Lupo. Shelves are the same

I used JVC 6x9's and ran the speaker cable's down the passenger side and under the plastic trim. Then wired them into the cars wiring loom ISO plug.
You would also need to make sure you have a headunit that has the rear speaker output (stock one only has front output unless its the upgraded version.
Also. If you take the rear side pannels off. You will find some pre cut speaker holes made to fit a mid and a tweeter on each side. If your careful you could cut the side pannels to fit speakers into those holes.