Adding Audi Sub to Concert radio?

[a4tdidriver]

I have a Concert radio with the standard speakers and no sub, I see that some avant models (I have a Jan 2001 avant) have an Audi sub in the boot, so was wondering if i can just add one into the standard system?

On the nearside of the boot i have the autochanger, but in the left there may be space for one? - Has anyone done this ?

Thanks