When you create a new core profile field, existing forms are not affected, only new forms that are created afterwards. If you have a customised form layout (a 2 column form for example), nBill would not know where to put the new field anyway, so it would be very difficult to do without causing complaints of 'nBill messed up my form!' - as such, I'm afraid you will have to add the field manually to each form, sorry. However, if the field name and contact/client mapping is the same as the core profile field, any updates you make to the core profile field will (optionally) be reflected on the order forms that use it. So if you just make it a very simple field, add it to each form (just setting the name and mapping and leaving everything else at the default values), then edit the core profile field to change the other properties, the modifications will be reflected on all your fields which should save a bit of time.