Hello,

You can customize code of invoice. You need to create your own module.

If you want to add any column then in .py file add following:

class account_invoice(osv.osv):
    _inherit = 'account.invoice'

    _columns = {
       #Fields you want to add
    }

#Function for your calculated columns(If needed)

account_invoice()

In .xml file inherit proper view according to your need. You can view many examples to inherit view in addons.

Thanks.