you can't.
I have no idea how the EB team does it (probably with the use of some hidden traits), but it seems that the Roman character's last names are all the same (romanvs). so regardless of whether he is a julii, claudia or cornelii, his last name would still be 'Romanvs'.
however, his first name may be modified as so when typed in the console, e.g. "SextvsA Romanvs" or "SextvsB" until "SextvsK", but you have to play a guessing game to figure out the letter that comes after their first name for the particular character.
also, there are certain characters (the starting ones and descendants of the starting characters, if I'm not wrong), that do not have a letter behind their first name for some reason. with the conflict of first names, only one character may be effected by console commands while the other fellow with the same name when typed in the console can't have anything done to him. such as Cnaevs Cornelius Blasio and Cnaevs Cornelius Scipio Asina when you start the game, "Cnaevs Romanvs" only effects Blasio.