Waterfall_13
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To give a hero 8 skills, you need to specify all 8 skills and the necessary perks in the hero's xdb file. They won't be visible in the skills window, but they will be displayed on the creatures, i.e., there will be Spirit, Luck, Attack, Defense, and everything else. This works 100%, I can even attach a screenshot.
As for making 8 skills upgradeable, I don't know. Someone did it by using HERO_SKILL_NONE 8 times; perhaps on a regular map, there will be an opportunity to upgrade all of them when leveling up. I haven't checked this for sure; I just saw that someone did it.
Added 1 minute ago
Regarding the "no refusal" aspect – I made 8 skills, but I reverted it to 5. In reality, it's not that interesting, at least in PvP or pre-made battles that don't involve hero leveling.
There are a total of 12.
The conversation was about something else, about being able to upgrade skills from zero without assigning all the hero's skills. I think I once came across a patch for the .Exe file to increase the number of skills to 8. But I don't remember where, and I guess the interface would have to be modified in some way for that.