I understand how flightgear outputs nmea data on port 5500 to Marble

but is there a way to pipe the the data regarding position of other players while online which i assume is also in the form of nmea data thats coming from a online server at mpserver02.flightgear.org to Marble?

so that i could track other players/traffic on my map as well?

Currently that's not supported. Do you have some more information about the data/protocol? I doubt it's possible to stream more than one source over nmea on the same port, but variants of it (use one port for each other plane, or some other protocol) might be possible.