Need to include custom RPMs to the CentOS 7 extracted ISO image

I am working on setting up a PXE based automated installation in our environment. As part of that, I want to install few additional packages via the kickstart file during the build. These packages aren't part of the standard ISO that CentOS provides (i.e.) it isn't available in the 'Packages' directory. So, I copied the required RPMs manually to the 'Packages' directory and ran 'createrepo --update .' but it didn't help. I looked around for any potential alternatives but couldn't find any.

I understand that I can install these packages via yum but I want them to be installed during the build and I don't want to mention these explicitly in the post install section as it defeats the purpose of having the %packages scriptlet in the kickstart file.

Any pointers on how to make the Packages directory in the CentOS ISO aware of the RPMs that I copied into it?

-Ram