I don’t think anyone has ever attempted to do this. You also need to make the content available in the database as a post type, or create a new search class file to directly go through the XML file, then after getting the results pass them through the SOAP API in the preferred format.
I’m not sure if there is a pre-made tool for this, but if there is, I would rather recommend using that. I would be a huge time saver for this task.