*Audi A3 8P Sportback 2009 Chorus unit speaker install help please*

You can install rear speakers in the stock locations in the rear doors, either get Audi speakers off ebay or aftermarket 6.5" components which will need fitting adapters (search ebay Audi MDF)
That's the easy bit
The biggest problem in your car is that you will have nothing to wire the speakers into. The rears run off a separate amp that resides in the stock subwoofer which you will also be missing. You can pick these up cheaply on ebay but you don't have any wiring in the car for this setup. You would need to get the original wiring loom from another vehicle or makeup your own which needs to run from the HU to the boot and then to the rear doors. Easiest option is to change the HU to an aftermaket one and just run new speaker cable to the rear doors (no Audi HU's have rear speaker outputs, they all need a rear amp).