Hi all,
here's what I found out at the dealer:
The radio is upgradable with a cd-changer.
It is rather expensive; necessary parts are.
- cd-changer
- mounting bracket
- connection set
- software download
- cargo area left interior panels (2)

This sums up to EURO 728.00, plus installation.

I think about the alternative to use an universal cd-changer,
one of those which connects to the antenna lead and emulates
an fm radio station. I'd place it where the original would also mount,
that means I'd have to buy the two interior panels. There's already
a power connector which could be used, and it should be possible
to slice into the antenna lead of the left rear side window.
The disadvantage would be that it's not possible to control the
changer through the radio, a separate operation panel has to be
placed somewhere.
Cost of this solution is about a third of the oem unit...

Best regards
Stefan