New Web applications are rapidly moving from stand-alone systems to distributed applications that need to interoperate with third party systems, such as external Web services or legacy applications. In most cases, this integration is not properly addressed by current Web Engineering proposals, that either achieve it only at one single level (user interface, process, code or data), or assume the existence of a central conceptual model, something which is not always true in a service-oriented scenario. This paper presents a model-based framework that provides concepts and mechanisms for facilitating the high-level integration of Web applications with third party systems, allowing the manipulation of the external entities of such systems as native elements of our models.