You can still directly edit the game's XML files if desired. But with the Expansions, the data within the base game files are often partially or completely re-written by the Expac, so changing the Base Game (ie, Vanilla) files will often seem to have no effect when running Rise and Fall or Gathering Storm. In order to be sure you are editing to the data loaded by the expac you would have to look into and make alterations as necessary in either of folder C:\Program Files (x86)\Steam\steamapps\common\Sid Meier's Civilization VI\DLC\Expansion1\Data or folder C:\Program Files (x86)\Steam\steamapps\common\Sid Meier's Civilization VI\DLC\Expansion2\Data