The vision of Industry 4.0 (I4.0) promises unprecedented interconnectedness of vendors, manufacturers and other partners in the value chain. A key enabler in this new landscape of communicating factories is the digital twin, a virtual replica of every asset that mirrors all its facets and properties along the whole life cycle. The ambition of including existing factories with legacy devices into this ecosystem is not without significant challenges. These older devices are equipped with purpose-built hardware and inflexibly bound to certain communication protocols that do not integrate with I4.0 interaction methods. In order to bridge this gap, an adapter is required that mediates between the highly optimized industrial protocols and modern, vastly linked I4.0 devices. This work presents a gateway, called the Multi-Message Broker (MMB), which can connect to legacy devices via a suite of appropriate interfaces and simultaneously offer I4.0-compliant APIs for these connected assets. This creates a digital twin for legacy devices and makes them accessible to I4.0 information exchange. The aim is to integrate plants and systems with the lowest possible effort. As existing device descriptions and information models can form a basis for the digital twin, this and their applicability for the MMB will also be discussed. A prototype that implements selected aspects of the concept will demonstrate the functionality and advantages of such a system.