as of v4.3.4.3 several filters have been added to add to the output

to add an additional “add to cart” button for example to the end of the form after all the ingredients groups , you can do so by adding the following to your themes function.php file

add_filter('wppizza_add_ingredients_filter_after_groups', 'my_custom_filter',10,2);
function my_custom_filter($str,$lbl){
$str.='<div style="margin:0 0 30px 0;padding:5px 3px 0 0;"><input id="wppizza-diy-to-cart" class="btn btn-primary" type="button" value="'.$lbl['add_to_cart'].'"></div>';   
return $str;
}

NOTE: as of “Add Ingredients” plugin version 5.0.9+ , the following should be used instead

add_filter('wppizza_add_ingredients_filter_after_groups', 'my_custom_filter',10,5);
function my_custom_filter($str, $labels, $itemId, $tierId, $sizeId){
$str .= '<div style="margin:0 0 30px 0;padding:5px 3px 0 0;"><input id="wppizza-alt-diy-to-cart-'.$itemId.'" class="btn btn-primary wppizza-diy-to-cart" type="button" value="'.$labels['add_to_cart'].'" /></div>';   
return $str;
}

to add some html elsewhere, there are also the following filters available you can use in a similar fashion:

• wppizza_add_ingredients_filter_before_form (to add something before the opening form tag)
• wppizza_add_ingredients_filter_form_start(to add something right after the opening form tag)
• wppizza_add_ingredients_filter_before_groups(to add something before the ingredients groups)
• wppizza_add_ingredients_filter_after_groups(to add something after the ingredients groups)
• wppizza_add_ingredients_filter_form_end (to add something right before the closing form tag – equivalent to wppizza_add_ingredients_filter_after_groups)
• wppizza_add_ingredients_filter_after_form (to add something after the closing form tag)