Copyright & Licensing
Creative Commons is a licensing scheme that allows authors to license their work so that others may re-use it without having to contact them for permission. You may not add a Creative Commons license to a work for which you do not hold the copyright. Find out more about Creative Commons Licenses
Adding a license to your work
If you retain the copyright, you can do whatever you like with your work, including adding a license to it.
Note that if you have assigned the copyright to the publisher, you will not be able to add a license to your work, including any earlier versions of it, unless you ask for permission from the publisher.
If your work contains third-party copyright material (see below), you must ensure that you have the copyright holder's permission to make their work available under a Creative Commons license unless an exception to copyright (such as quotations for the purpose of criticism or review) applies. For more information, see the Creative Commons Wiki.