Patching PS2 rosters

Hi there,
I wonder if someone could give me any tips about whot kind of "File Checksum" algorithm has been used for the rosters files.

I'm trying to patch the game on my PC but I can't make any of these files work if I don't solve the checksum issue.

I think I found the 4 byte the record the result of the checksum but I've not idea how they are calculated or in which section of the file...

Any suggestion appreciated, even abou what program I shoud use if I wanted to go further down on this topic

Thanks,
B