dunno. maybe there’s a plugin that does/allows that.
the wppizza registration function just uses wp’s standard functions
(although I don’t know why there would be anything different in a multisite installation, given that users are only ever registered in the parent site anyway)
furthermore, i would generally steer clear of messing around with such fundamental wp security stuff. those guys behind WP tend to know what they are doing and why they are doing it the way they are doing it ….