Hi SoftIntellect
Adding support for Purchase Orders PRO to ATUM's REST API is on our TODO list.
But, at this time, if you need to customize the REST API, it would require custom coding. And this is out of the scope of our support service, I'm sorry.
If you need this and don't know how to do so, I'd suggest you find a developer to assist you with this.
Best Regards,
Salva