first step:

you can customize it by inheriting base.contact template in XML, for example here is how to add additional field "my_field" just before address in address widget:

<template id="contact" inherit_id="base.contact">
    <xpath expr="//address/div" position="after">
        <div t-if="object.my_field and 'my_field' in fields" 
            class='css_editable_mode_hidden'>
            <div itemprop="my_field" t-esc="object.my_field" 
                style="margin-top:4px;margin-bottom:4px;font-size:10px;"/>
        </div>
   </xpath>
</template>
 

second step:

then in your qweb template, add "my_field" to fields list:

... t-field-options='{"widget": "contact", "fields": ["address","my_field","name",...],...

if your template is a new one, you need simply add my_field to "fields" list as shown above, if you extending some existing template, then you can inherit it and replace existing widget entry, using position="replace" property in xpath tag...