How to add modded mobs to a vanilla biome

I would like to add some modded mobs to the spawn pool of the desert biome, but when I tried using a desert.json it made me add every tag and overwrote everything, making the desert not spawn with cacti or other mobs. Is there actually a way to only add mob spawns to a biome without disrupting or overwriting what's already there, or disrupting what other datapacks do?