You'll need to know at least Some json to do this

Since 4.0.0, modpack developers can customize even more aspects of Additional Additions with behaviours. Behaviours currently can customize tool and armor stats.

What are behaviours? Those are json files that you'll need to write. Where do they go? In a datapack.

Structure of an example datapack that modifies the stats of gilded netherite chestplate

How to

All behaviours go in data/additionaladditions/behaviour folder of your datapack.

Materials: What and how you can customize

Those behaviours go in a subfolder named with the material name of the armor/tool material you want to customize.
For example, if you want to modify the stats of gilded netherite, create a folder called gilded_netherite inside the behaviour folder.
For example, if you want to modify the stats of rose gold, create a folder called rose_gold inside the behaviour folder.

Here are some properties:

Property Name	Applies to	Description	Value type
durability	Everything	Changes the item's durability	Integer
defense	Armor	Changes the armor item's defense/protection	Integer
toughness	Armor	Changes the armor item's toughness	Float
knockback_resistance	Armor	Changes the armor item's knockback resistance	Float
attack_damage	Tools/Sword	Changes the item's attack damage	Float
attack_speed	Tools/Sword	Changes the item's attack speed	Float
mining_speed	Tools	Changes the item's mining speed	Float

Those properties go into the json behaviour file you created.
Speaking of files, you'll also need names of them! Right, here they are

Behaviour/File Name	Applies To	Type (above table)
helmet	Helmets	Armor
chestplate	Chestplates	Armor
leggings	Leggings	Armor
boots	Boots	Armor
sword	Swords	Sword
axe	Axes	Tools
pickaxe	Pickaxes	Tools
shovel	Shovels	Tools
hoe	Hoes	Tools

In short:

1. In the behaviour folder, create a subfolder named with the material name you want to customize
2. Pick behaviour file name of the item you want to customize from the second table, and create it with the .json extension
3. Add some properties you want to customize from the first table, you'll need to know at least some json to do this!
4. Datapack should be ready!