Believe it or not there are decompilers around, the results vary though. Some would consider this "unethical". I am not sure myself which file in the mission is the map exactly, but I was looking at the same thing last night myself.
I was thinking of trying to port a map or two from SWAT 3 to the Source engine but I haven't got far as I can't find and decompile the maps, simply because I have no experience with such things.
I tried to do this; it's just not possible. Without a SCN to map converter, the only other way is to recreate all geometry within the map editor you are porting to. Take a look at how I got the cod4 map into swat 3, you basically will have to do the same. The only files in the swatdata.res and mission zip files that could be used in some porting method or converter tool will be the SCN files. for example missiona.scn.
Well that kinda sucks, SWAT 3 was my childhood and I have more mapping skills in Hammer than in Worldcraft, I really hoped this would be easier. I guess I can try and copy the geometry in Hammer but I will probably procarstinate and forget about it. But in theory it shouldn't be that hard to do, Hammer is just an updated version of Worldcraft, after all.
