Attempt to add new spells and skills.

What about this:

Table_CreatureAbility_CombatAbility

175 is the limit for the number of creature abilities... is it possible to add new functionality using scripts?

No, you can only create activatable abilities. I tried to create something like this from an existing spell: here, for example, a new zombie ability has been added - if the number of enemy units falls below 30%, the zombie gains a spell that, when activated, allows them to attack that unit regardless of distance.
Video
If the number of spells is expanded, it will be possible to create abilities like this, with normal descriptions and everything.