I tried duplicating an existing folder in opponents_avatars, renaming everything in the .opponents files, and adding custom assets, but John, Jamal, and Jong are still the only ones who appear when starting the game. I got it to work by renaming OG John's folder to John.bak and putting my custom assets in the new "John" folder. It works but feels like I'm doing it wrong, and of course it makes OG John go away.
I also figured out that I could add a new character to opponents_core and use them as a player character, but opponents_core characters evidently are assumed to always be female, which is not my intent.
No big rush but thanks for any help.