I'll copy and paste what I put in a reply to an e-mail to someone else:
Max files are just a compressed file which contains the PS2 files. In the case of PES it's 2 small icon files and the main data file. Now that you've got the correct OptionFile.LENGTH (= 1111040) the editor will un-compress the data file into OptionFile.data.
The PES data file consists of 11 blocks. Each block is structured as follows:
4 bytes data length
4 bytes checksum
4 bytes = x00 x0d x00 x00
data (first block is plain text, the rest are encrypted)
Next step is to update OptionFile.block (offset of the block's data) and optionFile.blockSize so that the editor can decrypt the data blocks. I suggest downloading this:
http://www.ps2savetools.com/download.php?op=viewdownloaddetails&lid=90
and export the main file, then search for x00 x0d x00 x00 in a hex editor.
You can check it's working by un-commenting the Optionfile.saveData method and also its use in OptionFile.readXPS. Running the editor and opening a file will now save the plain-text data to a file and if you open this in a hex editor you should see the structure of each block and stuff like player names.
Next step will be to update the static variables by looking at the file to find the offests, and also updating stuff like the list of nationalities, the generation of the GUI squad list, and the safety checking code of in TransferPanel (stops the user transferring a player to a squad which may crash the game in certain modes).
Hope that helps get people started, and apologies for my ugly code.