Me again,

after several tries I managed to do it like this:

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <record model="ir.ui.view" id="view_sale_order_form1">
            <field name="name">sale.order.form.inherit1</field>
            <field name="model">sale.order</field>
            <field name="inherit_id" ref="sale.view_order_form"/>
            <field name="arch" type="xml">
                <field name="pricelist_id" position="replace" />
                <field name="client_order_ref" position="after" >
                    <field domain="[('type','=','sale')]" name="pricelist_id" groups="product.group_sale_pricelist" on_change="onchange_pricelist_id(pricelist_id,order_line)" readonly="1"/>
                </field>
            </field>
        </record>
    </data>
</openerp>

If someone has some suggestions I would be glad to read them.