The implementation of numerical methods involving geometrical description of geosystems with the application of numerical meshes is of vital importance for investigation and modelling of complex, coupled, natural, or man-induced processes. Innovative application of CAD-Systems in combination with computer aided modeling of such systems provides a user-friendly powerful tool for the often complex geometrical descriptions necessary for comprehensive modeling of the dominant in situ processes. Here the development of such a CAD interface is presented, based on GOCAD, coupled with the software engineering of the finite element program RockFlow/RockMech and the implementation of polylines to describe complex geometrical structures and surfaces. This program allows the calculation of flow, transport, and deformation processes in one or more fluid phases in fractured and porous media. Such simulation is becoming increasingly important, especially in the fields of waste disposal, geothermal energy, and groundwater supply management. Here an example of the application of the CAD interface to RockFlow is presented for the modeling of flow during artificial stimulation (a 300-day long pump test) of a potential geothermal reservoir in the deep continental borehole (KTB) program at Windischeschenbach, Germany.