In some spanish forum found some reference of files and folders structure inside ps2 classics pkg`s:
The CONFIG file resides besides ISO.BIN.EDAT and ISO.BIN.ENC, PS2 classics GUI has a similar folder structure that is copied when building the pkg, but it seems to ignore additional files...
Found some references of how to create and encrypt config file, but I'm not sure if a config can be included with the pkg, seems like PS2 Classic GUI ignore additional files and there is no mention of a config file anywhere.