Hi Darkarturo,
The simplest way to do this is with a two page form and ask for any existing DNI on page 1, then you can load any existing data into the full form on page 2.
You can do it on a single page using JavaScript to make an AJAX query back to the server to get data. You would need some custom JavaScript linked to the DNI input. It would need to check that the DNI format is correct; request the data; handle the event that nothing is found; and handle the event that data is returned but the User has already added data to some of the other inputs.
Bob
but how do I do that bob, I need your help please, i need one example or help, I would be eternally grateful