New approaches are regularly developed, but their implementation in existing environments is a challenge, and Industry 4.0 is not different. In this paper, we present our approach towards a modular and adaptable gateway to retrofit brownfield installation for use in the context of Industrial IoT. The aim is to close the gap between existing operational technology installation and modern IT infrastructure. To fulfil time constraints and flexibility requirements, a highly decoupled architecture employing a streaming platform is proposed. For the data to be useful for downstream processes, it is annotated with means of Industry 4.0 modelling standards. The loosely coupled gateway presented by us is intended to help make existing installations Industry 4.0 capable without having to re-engineer them directly.