stock_picking_back2draft/views/picking_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record id="view_picking_form_reopen" model="ir.ui.view">
            <field name="name">view_picking_form_reopen</field>
            <field name="model">stock.picking</field>
            <field name="inherit_id" ref="stock.view_picking_form"></field>
            <field name="arch" type="xml">
                <xpath expr="//button[@name='action_cancel']" position="after">
                    <button name="action_back_to_draft" states="cancel" string="Back to draft" groups="base.group_user" type="object"/>
                </xpath>
            </field>
        </record>
    </data>
</openerp>