Context-aware computing is gaining much interest on mobile device platforms. This paper proposes a concept of Process-Based Composite Contexts in which a composite context can be easily composed of other contexts in form of a process. Developing such a context with Model Driven Architecture, we define PIM-, PSM-, and code-level metamodels as well as transformation rules between the meta-models. The composite context can then be ported to different platforms using only one PIM model. Also, we present a development of the composite context for two platforms - Windows Mobile and Android - using ATL transformation language.