I've never used RHEL, but from what I understand CentOS offers a complete replacement of packages/binaries. Just installing the CentOS repositories onto your RHEL system and installing CentOS and RHEL packages would, as I understand, cause conflicts and possibly an unusable system (as long as you don't install the same binary from both repos, you should be right, perhaps disable the centos repo by default and only enable when you want to install a particular package).
That said, the following link might be useful, the author seems to want to achieve a similar task to you:
http://cotdp.com/2012/07/installing-centos-packages-on-red-hat-enterprise-linux/
The information in the first comment of the above link might be most useful to you.