How to custom cento6 iso by replace the kernel package?

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http://serverfault.com – I want to custom cenos6 minimal iso by add some packages or replace some packages. for example, I want to replay the kernel package with kernel-xen rpm package, if I replace it directly, createrepo repodata xml files and make new iso again. When I use the new iso to install in my computer, finally, it will notice me that no kernel package will be installed and bootloader will the the default. I guess there must be one file which controlsl which packages will be installed but can't find in the original iso. maybe in the install.img? I search internet but can't get some useful information. Can (HowTos)