wizardz,
it's possible.
follow the below code.
account_invoice.py file
from openerp.osv import fields, osv
from openerp import models,fields,api
class account_line(models.Model):
_inherit = ['account.invoice.line']
product_position = fields.Char('Position')
product_note = fields.Char('Produkt Notiz')
account_line()
Now goto the xml file.
<?xml version="1.0" encoding="UTF-8"?>
<openerp>
<data>
<record model="ir.ui.view" id="account_line_inherit_view">
<field name="name">account.line.inherit.view</field>
<field name="model">account.invoice</field>
<field name="inherit_id" ref="account.invoice_form"/>
<field name="arch" type="xml">
<field name="discount" position="after">
<field name="product_position" />
<field name="product_note" />
</field>
</field>
</record>
</data>
</openerp>
you can see in print screen.
if you want that small module inform me.
You're about "Customer Invoice" or "Supplier Invoice"? each has it's own form! you'll need to inherit the right one in XML and add your columns in inherited record.