extra_accountinvoice_cobrador/account_invoice_view.xml

<?xml version="1.0" encoding="utf-8"?>

<openerp>
    <data>
        <record id="account_invoice_cobrador" model="ir.ui.view">
            <field name="name">account.invoice.cobrador</field>
            <field name="model">account.invoice</field>
            <field name="inherit_id" ref="account.invoice_form" />
            <field name="arch" type="xml">
                <field name="account_id" position="after">
                        <field name="employee_id"/>
                        <field name="factconcobrador"/>
                </field>
            </field>
        </record>
    </data>
</openerp>