API Design Using WCF

I am working on building an API (using WCF) that will take some of the internal e-commerce systems out to the outside world. The target users of the API are mostly small businesses that may not have extensive resources. To accommodate future API evolution, I decided that the API inputs and outputs would be strings (xml doucment converted to string). This got me thinking about all the possible ways that one day after getting strings in my methods and generating XML documents from them, how would I be able to map this XML document to strongly typed objects that I have access to - these objects are used to the internal system that my api provides.

What are the best methods to get an XML document converted to strongly typed classes (remember that I cannot directly convert xml to objects, as in some cases, I will need to add additional information to the api to build the objects that are needed by the internal systems)