I just booted up X-Com 2: War of the Chosen for the first time, but my character pool is empty, and there's nothing in the "Import Character" tab but a Developer Pool.
Where did my characters go?
Due to the major changes added in War of the Chosen (WOTC) there was actually a second folder added for the game in the
Documents\my games folder (path will be different on a non-windows machine).
Documents\my games\XCOM2 is for the base game, and
Documents\my games\XCOM2 War of the Chosen is for the expansion. If you take a look inside these folders you'll see a whole bunch of configuration files, including a separate character pool folder for both the base game and WOTC.
So in order to use your character pools you first need to create a character pool file for them by booting up the base game and navigating to the character pool page via the main menu. Once there you just check off all the characters you want to copy over, and then click "Export Selection". You can either add them to an existing character pool or create a new one. Once you've done that a character pool file will show up in:
Documents\my games\XCOM2\XComGame\CharacterPool\Importable (it's a .bin extension).
Copy over all the files you want to transfer to:
Documents\my games\XCOM2 War of the Chosen\XComGame\CharacterPool\Importable
After you copy the files boot up WOTC. Open up the character pool page, and click the import character tab. Then you just select the character pools you want to create, and then either import the entire pool or individual characters from the pool.