sale_product_multi_add/views/sale_view.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record id="sale_order_mass_import_form" model="ir.ui.view">
            <field name="name">sale.order.mass.import.form</field>
            <field name="model">sale.order</field>
            <field name="inherit_id" ref="sale.view_order_form"/>
            <field name="arch" type="xml">
                <xpath expr="//sheet/h1" position="before">
                    <div class="oe_right oe_button_box" name="buttons">
                        <button class="oe_inline oe_stat_button oe_right" name="%(action_import_product_to_sale)d" string="Products" Help="Select Products to Import"
                               type="action" attrs="{'invisible':[('state','not in', ['draft', 'sent'])]}" icon="fa-bars" widget="statinfo"/>
                    </div>
                </xpath>

                <field name="order_line" position="attributes">
                    <attribute name="options">{'always_reload':true}</attribute>
                </field>
            </field>
        </record>
    </data>
</openerp>