Geometric tolerances significantly influence the behavior of technical systems. However, tolerance effects are typically assessed in isolation and only weakly connected to system-level simulations used in model-based systems engineering (MBSE). As a result, tolerance-induced interactions across physical domains often remain invisible until late development stages. To address this gap, this paper proposes a general approach for embedding executable geometric tolerance simulations into system-level models using the Functional Mock-up Interface (FMI). Rather than focusing on data exchange alone, the approach enables tolerance models to actively participate in transient system simulations, allowing their effects to propagate across coupled subsystems. The main contribution is a method-independent concept for Functional Mock-up Units (FMUs) and a corresponding generation workflow that encapsulate complex tolerance simulations behind a standardized interface. A case study demonstrates how tolerance-induced forces can be efficiently integrated into a multi-domain system simulation without exposing internal model complexity.