Hi,

If you just want to add a new field to the res.partner model inheit the class in Python and add the field.

ClassResPartner(models.Model):

_inherit = 'res.partner'

field_new = fields.Char('field')

And to add it to the view you can use xpath,  this is a sample one,

<record id="view_partner_form" model="ir.ui.view">
            <field name="name">res.partner.form</field>
            <field name="model">res.partner</field>
            <field name="inherit_id" ref="base.view_partner_form"/>
            <field name="archh" type="xml">
                <field name="a_field_name" position="after">
                    <field name="field_new"/>
                </field>
            </field>
        </record>

Thanks