Salut!
For anyone who wants to play Hotseat (Multiplayer Campaign) - with the Historical campaign (superior), I finally figured how to do it
Quite a simple procedure if done correctly,
First - Cut and paste the contents of your imperial_campaign folder into a new one named default_imperial_backup.
Then move this folder outside the imperial_campaign folder if you hadn't already.
Then Cut or Copy the contents of the historical_campaign - located in 'custom' and paste them into the now empty imperial_campaign file .
That's it all done, now when you boot up the game - select 'multiplayer' and click on hotseat it will be the Historical Campaign!
Tip - in hotseat you can rename your faction ingame (so it's better to play Hotseat even if you are playing with 1 faction) - do this by clicking on your faction information tab, and where the name is click and you will be able to edit. I find this useful if you were playing, say England - and you migrated to Spain - hence you could rename yourself 'New Spain' etc.