Ususally there is no no map "decompiler", simply because the development team would never need one, they have the .MAP files and know everything they need to.
So, you will just have to hope someone comes up with one. I do recall bsp2map (or whatever it was called) for Half-Life, it was a bit buggy, the resultant .map usually was invalid in a few ways, but you could see how stuff worked, and that's not a bad thing
it's a GOOD thing...
Chris "DDP" Carter