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

Like I said, I'm trying to "add" an item to the loot table in the infernal fortress chest without overriding all the items, but when I created the loot table: chests/nether_bridge and added my item to the pool, it made it the only item that spawns in the chests in the fortress.

Is it possible to add an item from the mod to the standard vanilla loot chest table?

I would be grateful to all who help and a special thanks if someone will throw down an example for all this.