Workflow to change item permissions to approver and author


I'd like a workflow that will change the permissions on a document in a library to that of the author and workflow approvers. I have tried using the Impersonation step, but I cannot find a way in the SharePoint objects to get the approvers of a workflow.

For example, I'd like to be able to upload an expense report to a library. For people to be able to upload to the library, they must have permissions to the library. Once the document is uploaded, everyone should not be able to read the expense report. Only the author and the approvers should be able to.

Is it possible to change the permissions just on the document either with a workflow or code so that only the author and approval workflow approvers and have access to the document?
Thank you,