It's definitely possible to use a manual transmission on an engine that originally had an automatic transmission.
www.renntechmercedes.com does the conversion. Also, I remember there used to be a Brabus shop on Kifissias going towards Kifisia from Athens, a little before Ugeia. I don't know if it is still there, but your best bet would be to find the Brabus shop in Athens...they would be the best help.