I needed to install the libxml 2.7 package and for this i uninstalled yum (yum have dependencies on libxml), is there any way I can make the pass to work with yum rpm packages installed previously? I want him to do update of the packages installed via rpm before reinstalling the yum
It's a bad idea to uninstall yum (specially because all those dependencies will trigger more and more packages that need to be removed), but that does not break the RPM database, which still contains all packages you have installed.
If you need libxml 2.7 for a specific software, one option is to manually compile it and install in a different directory, then point the software to that libxml installation. Or you can build your own libxml RPM that install in a different directory.
As for how to install Yum again, you could take a CentOS DVD and
rpm -Uvh *.rpm, that could create a mess by installing RPM packages you didn't have previously though.