Well, the file you want is in the campmap/names folder -- DEFAULT_HEROES.txt (there's also a VIKING_HEROES.txt fileif you have VI). Scroll down past the Kings, and you'll find the default heroes for each faction. You can edit these to your hearts content.

A couple things to keep in mind:
1. The number after the faction name (ie. FAMOUS_HEROES:: FN_REBEL 18) is the number of heroes in the faction, so if you add or remove a hero, change this accordingly.
2. The date listed is the hero's birthdate I believe. They will not appear until about 16 years after that date.
3. ID_LIMBO means that they can appear in any of their faction's provinces. ID_*region name* means that they'll only appear as a high-end unit trained in that region.
4. The next number is their portrait number. -1 means they'll have a default, random portrait.
5. If you scroll down past the heros, you'll find a list of first and last names that each faction can use. If you pick a name that's not on this list, your hero's name will be gibberish. Adding/replacing a name also doesn't seem to work.

Back up the files and have fun