It's not earning the 10,00 WEN that is the difficult part - that's easy.
There are dozens of people working on the option file to make it correct to match the situation in real life footie transfers. Plus fixing flags and kits. It would take one person ages to do it all, and there are more teams and players than in PES2 now so.
It's rare for just one person to know what squad, formations and kickers etc that EVERY team in the game use. If you're going to do it yourself, best of luck to you.