product_ean13_purchase/purchase_order_ean13_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record model="ir.ui.view" id="purchase_order_ean13_view">
            <field name="name">purchase.order.ean13.view</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form"/>
            <field name="arch" type="xml">
                <xpath expr="//tree/field[@name='product_id']" position="after">
                    <field name="ean13"/>
                </xpath>
            </field>
        </record>
   </data>
</openerp>