Adding item to existing LootTables without overwriting

Hi there!

So, I'm well aware of how to make my own loot tables and even how to overwrite the vanilla ones to add in my custom items. But for the sake of mod compatibility, this isn't very much of a good idea.

I want to simply add an item of mine to appear in vanilla dungeon chests, how could I go about doing this without overwriting? Even if it requires a tiny bit of java, I don't mind. Just wondering if it's at all possible currently!