Can I confederate with factions in an existing campaign after buying the required DLC?

In my current campaign as Skaven (Clan Mors) I control most of the map. All other Skaven clans were either wiped out or merged into my rat-empire through confederation.

There is however still Clan Skryre and since I don't own the DLC required to play as them I'm currently not able to propose a confederation.

I don't want to kill my rat-bretheren but I'd also really like full control of the continent they're currently at so the easiest way to achieve this would be through merging them into my own faction. Also taking control of them would give me some helpful troops (DLC units) and their leader.

My question now is:
Will the option to propose a confederation show up in my existing save file if I buy the DLC or would i have to start a new campaign to see that option.

I'm also interested how this works with DLCs from Warhammer: Total War I since for the Mortal Empires campaign some factions are only unlocked through the respetive DLC from the first game.

warhammer-total-war warhammer-ii-total-war

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Can you Confederate DLC factions Warhammer 2?

You can't confederate the DLC factions. But every faction can use the DLC units and if you confederate those you get to keep the units, but you still can't make them yourself.

Can you Confederate other vampire Coast factions?

Tomb King and Vampire Coast factions cannot confederate.

Can you Confederate Averlorn?

You can. I confederated Avelorn while I was playing as Alith Anar (she was going to be killed). It gave me Allarielle and one of her hero, I couldn't get her a mount tho.

How often can you Confederate Warhammer 2?

Confederation combines the two factions into one, and cannot be reversed. Confederation incurs a public order and diplomatic penalty that will last five turns. You may not confederate again until this period ends, even if relations are good.

Ranking EVERY DLC For ROME II (2020 Tier list)

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Images: Kelly L, Julia Avamotive, Element5 Digital, Pew Nguyen