Add fields to the web service orders

[Totti]

When you request a specific order, and I am going to read the detail, I need to read the purchase price, how can I add this field?

<associations><order_rows nodeType="order_row" virtualEntity="true"><order_row><id>350</id><product_id>51</product_id><product_attribute_id>66</product_attribute_id><product_quantity>2</product_quantity><product_name>gfgfgfgfgfg : Cioccolato</product_name><product_reference>5445454</product_reference><product_ean13>0</product_ean13><product_upc/><product_price>107.762557</product_price><unit_price_tax_incl>62.540000</unit_price_tax_incl><unit_price_tax_excl>57.114155</unit_price_tax_excl>

I would like to add the field to the purchase price (wholesales).