I'm looking to add an item, mymod:music_disk_custom, to the loot table minecraft:chests/simple_dungeon. I first came across the subject of Global Loot Modifiers, but I want something other than a chance of my disc appearing in the dungeon chest. I'd like for my disc to specifically be one of the choices in the loot pool to which the vanilla discs belong because this reduces the likelyhood of those other entries appearing in addition to mine. This subtle balance is desirable for my case, especially if I were to add many new discs or other collectables. 

The first solution I thought of was to have a Loot Modifier which has a chance to add my disc(s), in addition to a proportional chance to remove the vanilla discs. However, I can't help but think modifying the loot table instead would be much easier and elegant. I found the LootTableLoadEvent and was able to get as far as:

@SubscribeEvent
public static void onLootTableLoad(LootTableLoadEvent event) {
    if (event.getName().equals(new ResourceLocation("minecraft:chests/simple_dungeon"))) {
        event.getTable().getPool("???"); // no methods to modify the entries :P
        // here, i would then add my item to the pool. easy p-z
    }
}

Immediately, I noticed there is no way to get vanilla loot pools, as it expects a name and yet only custom loot pools are given names. In addition, there are no methods to modify the entries of a loot pool. I'm curious why this seems to be discouraged? If adding pools to vanilla loot tables is allowed, why not modifying or removing vanilla pools? Should I continue with my original solution, or is there more to learn?

The global loot modifiers give you the generated loot as an input.

Surely for your feature, all you have to do is detect that something in that list is a music disc and then randomly replace it with your disk.

https://github.com/MinecraftForge/MinecraftForge/blob/42115d37d6a46856e3dc914b54a1ce6d33b9872a/src/main/java/net/minecraftforge/common/loot/IGlobalLootModifier.java#L74

6 hours ago, warjort said:

The global loot modifiers give you the generated loot as an input.

Surely for your feature, all you have to do is detect that something in that list is a music disc and then randomly replace it with your disk.

https://github.com/MinecraftForge/MinecraftForge/blob/42115d37d6a46856e3dc914b54a1ce6d33b9872a/src/main/java/net/minecraftforge/common/loot/IGlobalLootModifier.java#L74

Thank you for the response! This is simple and works perfectly for my case. There is a caveat that the chance of any of the new or targeted items appearing is fixed by the loot table's definition, which has a 32 in 129 chance of being a disc per roll. I'm fine with this, even though it is not the ideal behavior I hoped for in modifying a vanilla loot pool 's entries.