You can, but if you just installed the expansion you'd be missing A LOT of stuff it adds throughout the entire game. I would suggest starting a new game and using one of the new weapons to spice things up. But as Barry said, you can still join the Expedition after DC, and there are a few advantages to doing things in that order. At level 23 you'd be somewhat overleveled, but not too much, especially for the first time you visit Black Sea.