$sub = Ninja_Forms()->form( $form_id )->sub()->get();

foreach($fields as $field_id => $field_value){
    $sub->update_field_value( $field_id, $field_value );
}

$sub->save();

$sub_id = Ninja_Forms()->subs()->create( $form_id );

foreach( $form_fields as $field_id => $value ) {

    Ninja_Forms()->sub( $sub_id )->add_field( $field_id, $value );
}

$form_fields = array(

    $fiel_id_1 => $value_1,
    $fiel_id_2 => $value_2,
    ...

);