well u need a program, i forget the name, and it will decompile it for yas. only thing is just get a bunch if duplicate planess (rouge plugin) and its not very easy to work with if u are new to the whole mapping thing. if u just want to add some stuff, i would suggest spawning them through the script, not editing the map.
try to do a search in the forum for 'decompile' and there'll be plenty of information
just in case u were thinking that a decompiled map is a map to work with:
well, it isn't. u maybe can take a look at how things were done but the result of a decompile is ... crap!