make sure your mod's meta file has charactercreation set as the character creation level, save your root templates in public/mod/roottemplates
Now modding an existing save? I don't know if you can, but you absolutely can mod the campaign for a new game. I successfully added a small island full of orcs of cyseal
Note: it will crash the first time you load a savegame in a modded campaign, but after you restart it should work fine
edit: there's a guide with some tips here, most important is setting the campaign to be editable (step 2)
http://steamcommunity.com/sharedfiles/filedetails/?id=283846252]