purchase_add_products/purchase_view.xml

<?xml version="1.0"?>
<openerp>
    <data>
        <record id="view_order_form" model="ir.ui.view">
            <field name="name">purchase.order.form</field>
            <field name="model">purchase.order</field>
            <field name="inherit_id" ref="purchase.purchase_order_form"/>
            <field name="arch" type="xml">
                <field name="order_line" position="before">
                    <button name="%(action_purchase_add_multiple)d"
                            type="action" string="Activar Lector"
                            attrs="{'invisible':[('state','not in',['draft','sent'])]}"/>
                </field>
            </field>
        </record>
    </data>
</openerp>