Modding

Is it possible to modify skills and abilities in the game in any way? The defaultstats file doesn't help much since some abilities are missing there. And how did the creators of the RTF mod manage to overhaul the entire skill wheel, and so well at that?

.... in this version of RTF the wheel is old, but in ancient versions the wheel was indeed overhauled, though not entirely — only the magic schools

you probably mean Armageddon's Blade — there, one of the main features is the wheel with "new" skills, but in reality, Heroes has a problem with adding anything new due to the ubiquitous IDs hardcoded in the game code, but that's beside the point

the skills from the wheel are recorded in \GameMechanics\RefTables\Skills.xdb, first come the skills, then the perks, and it's the perks that get wrapped with additional effects

here's the Distraction ability (reduces the enemy hero's initiative) recorded like this (this is not the full part)

            <ID>HERO_SKILL_DISTRACT</ID>
            <obj>
                <Texture>
                    <Item href="/UI/H5A1/Icons/Abilites/Skills/128x128/Distract_Gray.(Texture).xdb#xpointer(/Texture)"/>
                    <Item href="/UI/H5A1/Icons/Abilites/Skills/128x128/Distract.(Texture).xdb#xpointer(/Texture)"/>
                </Texture>
                <CommonTexture/>
                <NameFileRef>
                    <Item href="/Text/Game/Skills/Common/Sorcery/Distract/Name.txt"/>
                </NameFileRef>
                <CommonNameFileRef href=""/>
                <DescriptionFileRef>
                    <Item href="/Text/Game/Skills/Common/Sorcery/Distract/Description.txt"/>
                </DescriptionFileRef>
                <CommonDescriptionFileRef href=""/>
                <SkillType>SKILLTYPE_SPECIAL_PERK</SkillType>
                <HeroClass>HERO_CLASS_NONE</HeroClass>
                <spellBuffs/>
                <BasicSkillID>HERO_SKILL_SORCERY</BasicSkillID>
                <SkillPrerequisites>

and here's the Defensive Formation ability

            <ID>HERO_SKILL_DEFENSIVE_FORMATION</ID>
            <obj>
                <Texture>
                    <Item href="/UI/H5A1/Icons/Abilites/Skills/128x128/Defencive_Formation_Gray.(Texture).xdb#xpointer(/Texture)"/>
                    <Item href="/UI/H5A1/Icons/Abilites/Skills/128x128/Defencive_Formation.(Texture).xdb#xpointer(/Texture)"/>
                </Texture>
                <CommonTexture/>
                <NameFileRef>
                    <Item href="/Text/Game/Skills/Common/Defence/DefensiveFormation/Name.txt"/>
                </NameFileRef>
                <CommonNameFileRef href=""/>
                <DescriptionFileRef>
                    <Item href="/Text/Game/Skills/Common/Defence/DefensiveFormation/Description.txt"/>
                </DescriptionFileRef>
                <CommonDescriptionFileRef href=""/>
                <SkillType>SKILLTYPE_SPECIAL_PERK</SkillType>
                <HeroClass>HERO_CLASS_NONE</HeroClass>
                <spellBuffs>
                    <Item>SPELL_SKILL_DEFENSIVE_FORMATION</Item>
                </spellBuffs>
                <BasicSkillID>HERO_SKILL_DEFENCE</BasicSkillID>
                <SkillPrerequisites>

I've highlighted in red the main distinguishing line — the buff, meaning you can attach a buff to a skill that will be applied to creatures, that seems to be how it all works, but I'm not entirely sure)) if anything, my knowledgeable colleagues will correct me