pos_customer_extra_fields/data.xml

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
        <template id="assets_backend" name="pos assets" inherit_id="point_of_sale.assets_backend">
            <xpath expr="." position="inside">
                <script type="text/javascript" src="/pos_customer_extra_fields/static/src/js/pos.js"></script>
            </xpath>
        </template>
    </data>
</openerp>