Make a template (start with your profile_add_body.tpl) with the items you want on your form and name it 'profile_register_body.tpl'
Then in includes/usercp_register.php
find
Code:$template->set_filenames(array(
'body' => 'profile_add_body.tpl')
);
replace with
Code:if ($mode=='register')
$template->set_filenames(array( 'body' => 'profile_register_body.tpl'));
else $template->set_filenames(array('body' => 'profile_add_body.tpl'));