I guess you could tweak Visual Munge to only compile those .lvl or just simple put in your file and have Vmunge compile it but like Teancum said, you don't need to extract anything, just use all the stuff they have already given us!
using the shipped assests that's a lot easier, thanks. Any idea on how to munge it though. I'm thinking just ticking the "shell" option when munging a world with those files in it.
What exactly are you trying to change? Shell editing isn't exactly for the feint of heart. In fact, I've compiled the shell before successfully, but you must be very careful what you change, or you might break online compatability (though admittedly some of the shell has nothing to do with online).