To add an existing book to a new series, go to the Books page and click on the first book in the series. Click on the Book Details tab.
Look for this link to create a new series.
This will give you the Series Management pop-up window where you can enter the series ASIN. Once you do this you'll see the book in its own series on the Books page. You can drag and drop the other books in the series into that series.
FYI, you can find the series ASIN on Amazon by heading to the first book in the series and looking for the Books In This Series section on the Amazon page:
The ASIN is in the url of the page and will usually come after the word "product/"
NOTE: If no books are assigned to a series, the series will not appear on the series list.