First off any wsdl can only have 1 target namespace. You can however import more namespaces by importing additional xml schemas. But mendix does not allow you to customize your published wsdl. I guess that you could publish a custom request handler that validates incomming xml messages and calls a xml to domain mapping to directly map your incomming request into mendix. That way you wont have to parse the xml manually.
Just out of curiousity, why would you want to add additional namespaces? It is your webservice, you are publishing it so why add the complexity of extra namespaces?