Addition to the Magento web service

We are developing a Magento webshop which should interact with our Navision ERP program. For the connection we use and external connector "NavConnect". We need to pick up a purchase number, filled in by the customer at the check out and we have to get this into our ERP software. Our connector is already ready to pick up the PO number, but the web service of magento should be modified as well so that it can be picked up. We need a programmer to do this. This should be a minor modification.