This paper presents a concept for integrating geometric tolerance analysis into system simulations using Functional Mock-up Units (FMUs). While various methods and tools for tolerance analysis exist in the mechanical domain, there is currently no standardized or widely established approach for their integration into multi-domain system simulation. This work proposes a structured FMU interface based on the Functional Mock-up Interface (FMI) standard, enabling a modular and reusable representation of tolerance behaviour. The concept is demonstrated in a case study, in which an FMU for static tolerance calculation was implemented and successfully verified against a commercial analysis tool. Furthermore, the use of the FMU with FMUGym illustrates the potential of the FMI ecosystem to flexibly combine tolerance models with other simulation environments and analysis methods. The results demonstrate that FMUs can provide a suitable and tool-independent interface for integrating geometrical tolerance effects into system-level simulations and model-based engineering processes.