hi you can replace existing page with your own code

You can see purchase order line in:

Purchase / Invoice Control / On Purchase Order Line

Best Regards

yes..this is the form view...located in purchase module in th file purchase_view.xml

<record id="purchase_order_line_form" model="ir.ui.view">
            <field name="name">purchase.order.line.form</field>
            <field name="model">purchase.order.line</field>
            <field name="arch" type="xml">
                <form string="Purchase Order Line" version="7.0">
                    <sheet>
                        <group>
                            <group>
                                <field name="product_id" on_change="onchange_product_id(parent.pricelist_id,product_id,0,product_uom,parent.partner_id, parent.date_order,parent.fiscal_position,date_planned,name,price_unit,context)"/>
                                <label for="product_qty"/>
                                <div>
                                    <field name="product_qty" on_change="onchange_product_id(parent.pricelist_id,product_id,product_qty,product_uom,parent.partner_id,parent.date_order,parent.fiscal_position,date_planned,name,price_unit,context)" class="oe_inline"/>
                                    <field name="product_uom" groups="product.group_uom" on_change="onchange_product_uom(parent.pricelist_id,product_id,product_qty,product_uom,parent.partner_id, parent.date_order,parent.fiscal_position,date_planned,name,price_unit,context)" class="oe_inline"/>
                                </div>
                                <field name="price_unit"/>
                            </group>
                            <group>
                                <field name="taxes_id" widget="many2many_tags" domain="[('parent_id','=',False),('type_tax_use','!=','sale')]"/>
                                <field name="date_planned" widget="date"/>
                                <field name="account_analytic_id" colspan="2" groups="purchase.group_analytic_accounting"/>
                                <field name="company_id" groups="base.group_multi_company" widget="selection"/>
                            </group>
                        </group>
                        <notebook>
                        <page string="Notes">
                            <field name="name"/>
                        </page><page string="Invoices and Receptions">
                            <field name="invoice_lines"/>
                            <field name="move_ids"/>
                        </page>
                        </notebook>
                    </sheet>
                </form>
            </field>
        </record>