I'm writing a modul, now I can add a field like this:

<record id="sale_order_form_ext_view" model="ir.ui.view">

<field name="name">sale.order.form.ext</field>

<field name="model">sale.order</field>

<field name="type">form</field>

<field name="inherit_id" ref="sale.view_order_form" />

<field name="arch" type="xml">

<xpath expr="//field[@name='carrier_id']" position="before">

<field string="Liefertermin" name="delivery_date" placeholder="Liefertermin"/>

<field string="Lieferung" name="delivery" placeholder="Lieferung" />

</xpath>

</field>

</record>

But when I want to put it under the order_policy, it doesn't work : 

<record id="sale_order_form_ext_view" model="ir.ui.view">

<field name="name">sale.order.form.ext</field>

<field name="model">sale.order</field>

<field name="type">form</field>

<field name="inherit_id" ref="sale.view_order_form" />

<field name="arch" type="xml">

<xpath expr="//field[@name='order_policy']" position="before">

<field string="Liefertermin" name="delivery_date" placeholder="Liefertermin"/>

<field string="Lieferung" name="delivery" placeholder="Lieferung" />

</xpath>

</field>

</record>

Why is that so difficult? The field order_policy is at "other information" but on the same sale_order