Creating an RPM package with configuration files based off of another package's configuration files

Okay so I’m making a “spin” of Fedora and I want to tweak the default user configurations, grub settings, and user shell in an RPM package. However, the only way I can modify the latter two is by changing files installed by packages like grub2-tools and shadow-utils with sed in my make install script. Because the RPM build root does not include these files/packages, the build fails. Is there a better way to do this? I do not want to change other grub defaults like the resume drive or anything else generated by the installer. I could also just put this stuff in a post script for the lvecd builder but I would prefer a package for this. Thanks

   If you’re didn’t already, don’t forget to ask on the Spins IRC-channels (like a #fedora-kde).  They should know for sure.
PS:  join Guix, we’re love to thinker configs :)