If you want to add any item to the treasure pool, you will need to add some lines to the item's def:
treasureCommonality sets the chance of the item to appear as a treasure. The higher the number, the higher the chance.
treasureAmountPerCell sets how many of the item will generete at a treasure spot. If not set it will default to 300.
deepCountPerPortion sets how many of the item will be dug up once deep drill progress is 100%. If not set it will default to -1, meaning the item will never run out, so it is important to set this property.
You can look for more examples in the mod's defs and patches.