Hi,

For adding a new field in purchase order line, try the below code

in python

class PurchaseOrderLine(models.Model):
    _inherit = 'purchase.order.line'
    your_custom_field = fields.Char(string="Custom Field")

in XML

<record id="purchase_order_form_inherit" model="ir.ui.view">
    <field name="name">purchase.order.form.inherit</field>
    <field name="model">purchase.order</field>
    <field name="inherit_id" ref="purchase.purchase_order_form"/>
    <field name="arch" type="xml">
        <xpath expr="//field[@name='order_line']/tree/field[@name='field_name']" position="after">
            <field name="your_custom_field"/>
        </xpath>
    </field>
</record>

Regards