Lindsay is correct, this is a question the Service Desk team will need to address. Basically, if it can be done through the Service Desk API, it can be done within Process Automation.
I did look into the Service Desk reference guide and am not seeing any soap calls for this, furthermore I found this:
Technical Reference Guide Release 12.9.00
In a CA SDM workflow, a ticket with a running CA Workflow or CA Process Automation instance cannot be closed. The Accumulate, Expedite, and Insert Task functions are disabled for tickets using CA Workflow.
Which to me indicates that this will not be possible through the API.
I have moved this to the Service Desk community for further assistance and clarification.