Hi,
So you want to add custom item fields between the characteristic dropdown fields, right ?
If it is, I think that the easiest solution will be to edit the view "product / show_block_characteristics" and replace:
echo $this->characteristic->displayFE($this->element, $this->params) . '</div>';
return;
By:
$html = $this->characteristic->displayFE($this->element, $this->params) . '</div>';
// Your code to add the field values
echo $html;
return;
And add custom code before the "echo $html;" to replace the html by the custom item field code which can be found in the view "product / show_block_custom_item".
And in the view "product / show_default" remove the code:
//LAYOUT show_block_custom_item
$this->setLayout('show_block_custom_item');
echo $this->loadTemplate();
which is displaying the custom item fields on the product detail page by default.
I think that you have all the required informations, now it require PHP knowledges to adapt that custom code