Making a Redhat/CentOS .iso is extremely complex and there is very little documentation from Redhat, or anyone else, on how to do it.
We have our own semi-automated system here but it is still hard work to create a new version. If you are really interested then I suggest you start with Google. There is also an oldish document on here somewhere which shows you how to "break-in" to an SME .iso to add your own modifications/RPMs to it. It's not complete but it will give you a start point.
You cannot incorporate yum updates as such, the .iso is a collection of RPMs at a point in time. This is why neither we, nor SME Inc, nor anyone else, produce .ISOs very often. The idea is you take the last available .iso and then yum update it to the current release. Because the .iso is rpm based, you might want to learn a bit about rpm before you start. Again, there is documentation on Contribs regarding the creation of rpms.
Sorry to sound overbearing or patronising but this stuff is really quite hard work and requires a lot of hours to put together.
Kind Regards