Try looking at this Removing files from & repackaging CyanogenMod ROM zip files. It doesnt deal with Odin, but provides you with valuable working information. You should be able to use an archive program on PC and recompress to .tar, .gz, and others. Dont forget to re-sign the archive. Keep a copy of your untouched ROM until you have the modified ROM tested and running.
The ROMs are signed with a signature. When repacking if they don't get signed they will fail in flashing. This link will help explain it.
So basically after downloading the firmware you have to rename it so you can extract the files, change them as you need, recompress them again and re-sign them, so you can flash them using Odin. Its as simple as that.