Add items to vanilla chest loot tables without overriding the whole chest

As the title said, I'm trying to 'add' an item to the end city chest loot tables without overriding the entire thing, because when I made the loot table:

• chests/end_city_treasure

and added my item to the pool, it made the modded item the only thing that spawns in the end city chests.

Isn't there any way to ADD a modded item into the default vanilla loot table? I really appreciate any help you can provide.