You can add the vocabulary from any of the lessons to your Word Bank by going to the lesson page and selecting Lesson Materials. Under the Vocabulary heading you can choose which words to add to your Word Bank.
You can also add words from the Core Word Lists to your Word Bank. In the Japanese Resources menu (in the red bar at the top of the page) select 2000 Most Common Words. You'll immediately see that there's a button there for adding words to your Word Bank.