After the first wave of hacks/model editing Nexon put a Xor encryption on most of the hfs, also (it might be hackshield I'm not sure on this) but I've read that something checks the integrity of the files as well (Also may have just been when the game updates)... They have a mod to help edit some more hfs called good things lite they use it for model editing so I'm sure at least a little bit of it would apply to a few special hfs files. Generally though the issue isn't changing it from hfs to zip, it's changing them back to hfs while maintaining the integrity (Xor encryption + data)
EDIT: But to be honest I'm still extremely new to this as well... I've been here 6 days as of today, so I'm probably not the best one to ask. I'm just a relatively quick learner, and I've been a part of quite a few different hacking communities.