The transformation from linear production processes to a circular economy is gaining increasing importance due to the necessity of reducing resource consumption and extending product life cycles. In this transformation process, particularly for Manufacturing Execution Systems (MES), requirements arise to adequately and efficiently support circular processes. This paper demonstrates how a MES system must be adapted or designed to meet these requirements without losing the necessary flexibility and adaptability. An essential aspect of the approach is integrating existing IT systems (e.g., ERP systems), which are not yet configured for circular processes due to their limited adaptability and rigid functional and data structures. Through a design science-oriented approach, a conceptual extension for the circularity of an existing MES system is developed and prototypically implemented for a circular process. This generally illustrates which functions and data structures the system must extend or adapt. In the context of learning factories, this shows students the functionality of a circular system and demonstrates the connectivity to existing legacy systems, thereby ultimately fostering the competence to design these systems to be circular in practice.