I e mailed the question to the factory as to whether Flarm could work with Transponder equiped aeroplanes.
here is their answer;
It is off course possible to combine FLARM and an transponder receiver, but PCAS (transponder receivers) have such a poor performance in busy airspace (where FLARM is most often used) that we are not sure if this is really desirable…
so, looks like a common system is here already
anyone for FLARM as an add on to their Transponder?