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Done. In the Sovereign (AdvMapHeroShared) file, I wrote in HERO_CLASS_DEMON_LORD_1, and in UIGameRoot (UIGameRoot), I added what is highlighted in the screenshot below.
If needed, we can create a mod for the mod that unlocks campaign heroes.
And another question: will changing the "class" parameter in the Sovereign file affect anything else? If so (in a significant way), then let's forget about it.