I did put Ride the Lightning in for Creeping Death (for a friend)...so Ride the Lightning plays while electric chair multi-ball is playing.
I honestly don't see how you would add one song without removing another.
It is nice and saves time that the small snippets of the songs are taken from the locations that have the entire songs located, but it also makes it less flexible.
I believe I have tried to change a small snippet also and even that that injects that change into the entire song locations.
One thing I really don't understand is why there are 2 locations for each entire length song. It seems that they both behave the same way and replacing one automatically changes the other. They are different locations and also different requests. My first guess was that one was for the game and the other for the jukebox, but since they are tied together (if one is replaced they both get replaced) it seems weird that they even have it in 2 different locations.
Good luck...but I don't see how it could be done.