function my_import_stuff( $user_id, $user_meta ) {

    $data = array(
        'user_id' => $user_id,
        'user_registered' => date( 'Y-m-d H:i:s', strtotime( $user_meta['user_reg'] ) ),
    );

    wp_update_user( $data );


}

add_action( 'wpf_user_imported', 'my_import_stuff', 10, 2 );