How to bring workflow to Purchase Order approval

I have a process where, all the Purchase Orders are sent to an approval authority for approval after which, the Order is placed on Supplier. Is there a workflow for Purchase order?? Or if not then how to create a workflow for PO.?

Which version of AX it is? AX 2012 has purchase order workflow out of box, you just need to Activate the change management, configure the workflow and use it.

I am using AX 2009