To add pages mid book, 

1. (if you need to) you will add more pages by going to Manage < Manage account 



Scroll down, click on the number < select new quantity < hit UPDATE information

 

Adding pages will automatically place them all at the end of your book.

If you do not need to add pages, you will just need to move your blank pages around in the next step.

2.  Moving Pages

In the designer, across the top toolbar you will find “move pages”
IMPORTANT NOTE: Only Admin users will have access to this unless you adjust permissions under Manage Permissions

 

To move in spreads: Click, hold and drag your spreads around to move your blank pages to correct location and make sure to hit Confirm moves!

 

To move single pages: Scroll down to the bottom of this Move Pages window, < click Advanced Page Moves < click hold and drag single pages and hit "Commit Moves"" when complete.

 

Now your pages should be in the right place!

You can move pages as many times as needed before submitting for final production.