If you are on our Plus plan, you can import your own vocab. 

To import your own vocab go to My Content > Flashcards > New Flashcard Set

Once you've created it, you can tap on "+ New Words" and add new words to it.

The only caveat is that if a word you entered doesn't appear in any of our videos (and therefore doesn't yet exist in our database), then you won't be able to add it. But this is very rare.