Even after the advance of model driven engineering, reusable tool integration remains one of the greatest software engineering challenges. As we venture toward generic heterogeneous modeling tool interoperability, we focus on the most commonly used application programming interface (API)-level tool integration. In this paper, we propose a unifying model transformation bus. It is a model-driven framework utilizing multi-paradigm modeling (MPM) techniques which aims towards automated interoperability realized from a specification. We demonstrate an MPM specification for integrating model transformations engines using their APIs while orchestrating method calls and data conversions. Finally, we discuss the implications of such system, its benefits, limitations and future use.