In the past with Mac OS 10.1, it was really hectic adding extra keyboard scripts to the OS. It required changing the contents of a very deeply buried Localized.rsrc file and converting back and forth between Data fork and Resource fork and then the thing worked.
In Jaguar it is a lot easier. You simply need to take your Mac OS 9 keyboard script and add a .rsrc extension to it and then drop it in the /Library -> Keyboard Layouts or ~/Library -> Keyboard Layouts folder. Log out, log back in and it all works.
So, if you need to add another keyboard script to the system that Apple doesn't provide (Russian phonetic/transliterated in my case), you can do it without any problems.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020824214409662