You can add reviews from other sources via the Editorial Reviews option available through AuthorCentral.
You have to first create an account if you haven't already. Then you'll need to add your books to your bibliography. Then you'll be able to add editorial reviews to your book(s).
You can learn more about what you can do with your AuthorCentral account here
If you've worked with us, let's assume you already have an Author Central account.
If not, you can create one here.
Within your AuthorCentral portal, you can update your book descriptions, author page, and check sales (print only).
If there's an issue with your Author page or book listing on Amazon, there is a support team at Amazon available to assist you. Believe it or not, you can actually talk to a real human being! They can help you resolve a lot of common problems.
Note: Any changes made in your description via your AuthorCentral account will override changes at CreateSpace or via KDP.
If the link doesn't work, look for the Contact Us tab somewhat hidden at the very bottom of the screen after you login:
It's a good idea to check your own listings now and then (and your Author page) to ensure nothing has gone astray.
Hope that helps!
These slides were in a presentation I gave back in 2016, so it might be a little dated... but most of it should still apply: