Hi,
First you need to create custom fields of the table "product" and not "item".
Then, as there is no option to display the fields on the listing for now, you need to edit the file "listing_table" via the menu Display>Views and add the HTML for the title of the column, like the other column titles there, and then you need to add the td for each product in the table.
To display the value of a custom product field in a cell of the table, it can be done with such code:
<td>
<?php echo $this->row->COLUMN; ?>
</td>
where COLUMN is the column name of your custom field.