It is possible to convert XML-RPC to a DOC-LIT WSDL file. Here is a decent tutorial on how to accomplish this: http://webservices.xml.com/pub/a/ws/2002/12/18/endpoints.html
Once converted to a valid WSDL file it should import to a relational or web service model without any issues.
You can do XML-RPC with Teiid as well, but it requires that you have an XSD, or you have to do some tricky hand modeling. I have also been considering adding some other ways to generate a model, such as Relax NG, or a XML example doc.
Do you have any kind of contract for you service?