You should create a new module, which inherits the new fields (account_invoice__new_fields.py):

class account_invoice(osv.osv):
    _columns = {
        'field1': fields.char('Field1', size=128, help='Field 1 test'),
        'field2': fields.char('Field2', size=128, help='Field 2 test'),
    }

And then in view put them in right position(account_invoice_new_fields_view.xml):

<?xml version="1.0"?>
<openerp>
    <data>
    <record model="ir.ui.view" id="account_invoice_new_view"
        <field name="name">account.invoice.new.form</field>
        <field name="model">account.invoice</field>
        <field name="inherit_id" ref="account.invoice_form"/>
        <field name="arch" ref="xml">
            <field name="fiscal_position" position="after">
                <field name="field1"/>
        </field>
    </record>
    </data>
</openerp>

That should work for you (add also some __init__.py and __openerp__.py