I believe that can be done using map.ini. In Gladiator Underworld (and its sequel) by jbv3737, he wrote a lengthy code to modify the appearance, skills and powers of three game heroes. I don't know about the name, perhaps it may require deeper modding.
This, I suppose, is the only way to add custom heroes: modify existing game heroes.
Hope someone else answers you, but till then you should give the map.ini files of the above mentioned maps a read.