I've tried but always failed to hack ePSXe save states.  I knew to change it to zip, but after I've unzipped, edited, and rezipped the emulator doesn't recognize it.  Am I zipping in the wrong format?  What am I supposed to use?

The savestates are compressed in the GZIP format, which can compress only one byte stream. ZIP is similar to that, but adds some internal structure to allow storing several files with all their attributes in it.

The GZIP library can handle uncompressed files, so (as KingMike said) you don't need to recompress them.