How do you add payables/receivables to a Smart Project using the Home Portal API?

We have a working API  to create Smart Projects, but we would like to add a payable (interpreter cost) and a receivable (client charge) to the Finance section.

We have been told in a previous XTRF ticket that this is possible, but I can't find any documentation. 

Does anyone know how to do this? Or point me at the documentation?