Is there a web service to add an assignee to a requistion approval workflow ?

Summary:

For certain requisitions that are created by web service from another application, we would like to add an extra assignee to pre-approve the requisition.

At the moment we have to do this manually, but we would like to automate it.

Is there a web service where we can add a pre-approver to the requisition approval workflow automatically after the requisition is created ?

Content (please ensure you mask any confidential information):

Here is how we add the pre-approver manually , but we would like to be able to do this automatically by web service.

Version (include the version you are using, if applicable):