Description of the requested features
Allow existed loot_tables (such as shipwreck, ruined_portal or moded loot_tables) to be used in addition to configured champion loot tables.

Describe why you want this. If there's a reason.
While modpack compilers can manually modify the drop of the champion loot, it become quite frustrated and time-consuming when there are a lot of mods. Also, some mods may implicitly inject their loots into the loot tables without notices. This might cause some of the dungeon loot to go slip during manual configuration. Therefore, allow loot tables to be used for champion drops would considerably make configuration much easier and add compatibility with mods that modify loot_tables.

You can already do this. Set champions to use a loot table, grab the data/champions/loot_tables/champion_loot.json and override that with a datapack, you can then include any arbitrary amount of other loot tables as nested loot table pools within those files. I'm doing this currently in FTB Inferno so if you need an example you can grab that from the ftb app and see how to do it.

Adding to the additionalLootTables would be a separate thing but really not necessary as is from a pack dev standpoint.

Are you suggesting something like a config option additionalLootTables = [] where you can add the resource location of the specified loot tables?

Yes, exactly!