You can only play the expansion itself in the standalone apps. It's intended you enable the expansion in the main game and you never actually play the standalone apps. (There are many reasons for why the expansions are standalone and why you can start and play a character in them. And some just like to play that way, but most people of course prefer to play with their original character(s).)
So, do just that. Enable the expansion in the main game, then you can go to/from the expansion with any character you've created. (If you are on iOS, you can even delete the expansion app, if on Android, please see this:
viewtopic.php?f=3&t=4669)