In Odoo first, you have to install accounting module and in our system add existing fields and inherit the accounting module in our own module.

We will follow these step given below:

Step1- First we create our own module and inherits the account.invoice.line object and also inherits the account.invoice object then add our existing fields in an invoice.py file, and also add this invoice.py file in __init__.py file.

Use this code show in below

import json
from lxml import etree
from datetime import datetime
from dateutil.relativedelta import relativedelta

from openerp import api, fields, models, _
from openerp.tools import float_is_zero, float_compare
from openerp.tools.misc import formatLang


class AccountInvoice(models.Model):
    _inherit = 'account.invoice'  
    @api.multi
    def get_taxes_values(self):
        tax_grouped = {}
        for line in self.invoice_line_ids:
            price_unit = line.price_unit * (1 - (line.discount or 0.0) / 100.0) + line. yourfield
            taxes = line.invoice_line_tax_ids.compute_all(price_unit, self.currency_id, line.quantity, line.product_id, self.partner_id)['taxes']
            for tax in taxes:
                val = {
                    'invoice_id': self.id,
                    'name': tax['name'],
                    'tax_id': tax['id'],
                    'amount': tax['amount'],
                    'manual': False,
                    'sequence': tax['sequence'],
                    'account_analytic_id': tax['analytic'] and line.account_analytic_id.id or False,
                    'account_id': self.type in ('out_invoice', 'in_invoice') and (tax['account_id'] or line.account_id.id) or (tax['refund_account_id'] or line.account_id.id),
                }

                if not val.get('account_analytic_id') and line.account_analytic_id and val['account_id'] == line.account_id.id:
                    val['account_analytic_id'] = line.account_analytic_id.id

                key = tax['id']
                if key not in tax_grouped:
                    tax_grouped[key] = val
                else:
                    tax_grouped[key]['amount'] += val['amount']
        return tax_grouped

class AccountInvoiceLine(models.Model):
    _inherit = "account.invoice.line"
    _description = "Invoice Line"
    yourfield = fields.Integer(string='Your fields name', default=10) 
    
    @api.one
    @api.depends('price_unit', 'discount', 'invoice_line_tax_ids', 'quantity',
        'product_id', 'invoice_id.partner_id', 'invoice_id.currency_id', 'invoice_id.company_id')
    def _compute_price(self):
        currency = self.invoice_id and self.invoice_id.currency_id or None
        price = self.price_unit * (1 - (self.discount or 0.0) / 100.0) + self. yourfield
        taxes = False
        if self.invoice_line_tax_ids:
            taxes = self.invoice_line_tax_ids.compute_all(price, currency, self.quantity, product=self.product_id, partner=self.invoice_id.partner_id)
        self.price_subtotal = price_subtotal_signed = taxes['total_excluded'] if taxes else self.quantity * price
        if self.invoice_id.currency_id and self.invoice_id.currency_id != self.invoice_id.company_id.currency_id:
            price_subtotal_signed = self.invoice_id.currency_id.compute(price_subtotal_signed, self.invoice_id.company_id.currency_id)
        sign = self.invoice_id.type in ['in_refund', 'out_refund'] and -1 or 1
        self.price_subtotal_signed = price_subtotal_signed * sign

Step2- Then we have to create an invoice.XML file in our module and inherits the view of account.invoice.line object and then add it to our existing fields in this file and pass this file in __openerp__.py file in module.

Use this show in given below

 <record id="invoice_form" model="ir.ui.view">
		<field name="name">account.invoice.form.inherit</field>
		<field name="model">account.invoice</field>
		<field name="inherit_id" ref="account.invoice_form" />
		<field name="arch" type="xml">
			<data>
				<field name="quantity" position="after">
				<field name="yourfield"/>
				</field>
			</data>
		</field>
     </record>