1) You can add it to the %post section of the kickstart file. For
example you can do:
cat >> /path/to/file << FOE
text to add to file
more text
FOE
That's the right way to do it for small changes (one or two files, or
minor edits to an existing file).
If you want to include a whole load of files, you really need to build
your own RPM (a bit of a pain), create a local repository (dead easy),
and include that package in your kickstart file.
James