Manipulating registration data in a preHook snippet

In my registration form I have to fields "first name" and "last name". I don’t want the user to enter his fullname again. So I tried to use a preHook snippet which will generate the fullname property out of these two fields:

$firstname = $scriptProperties['fields']['firstname'];
$lastname = $scriptProperties['fields']['lastname'];
$scriptProperties['fields']['fullname'] = $firstname.' '.$lastname;

Unfortunately the user’s fullname isn’t set to the generated value.

$fullname = $hook->getValue('firstname').' '.$hook->getValue('lastname');
$hook->setValue('fullname',$fullname);

By the way how can I avoid saving the first and last name in extended fields? Is there away to provide a kind of "ignore fields list"?