If you are including the vB backend you can make use of the datamanager
https://vborg.vbsupport.ru/showthread.php?t=82836
I think you can add extra fields by something like this:
PHP Code:
// $userdm = data...bla...bla...
$userdm->validfields['fieldname'] = array(TYPE_STR, REQ_NO);
$userdm->set...
Although not directly link to you original question there is more info on adding a field to a datamanager here:
https://vborg.vbsupport.ru/showthread.php?t=119376
Someone might need to verify it but its worth a try