Invoke PostgreSQL user defined function into invoice module to generate invoice no.

Hi, I m on Windows 7 and OpenERP 7.0. I want to generate invoice number as per my defined criteria. For that i have made one function in PostgreSQL which returns invoice no. like "SAJ/13-14/0001" and so on... Now i want to use it in invoice module. Can i configure it in following configuration section. "Settings" => "Configuration" => "Invoicing" and then in the **"eInvoicing & Payments "** section. If answer is yes then how ?? Or else is there any other way to invoke it using python code ??? Or should i have to make addon ?? BTW, Sorry for BAD English. Thanks...