You make it sound easy.
You would need to decompile android.policy, services.jar, framework-res.apk on both roms.
Then you would have to dex-to-jar the necessary lockscreen smalis to view them as readable code.
You would then have to kang, or rewrite the lockscreen code in java.
Then you would have to convert the java smali back to dex.
Then you would have to get into framework-res.apk and add the new images, then add the values to the corresponding xml files.
Then recompile everything and pray that when you reinsert the files your phone doesnt throw up all over the place.
Im sure theres more, but thats it in a nutshell.