The radiation transport simulation of complex nuclear facilities, such as ITER, involves the process to adapt the highly complex engineering CAD models to radiation transport computational codes. An essential feature of this process is the generation of void space. In this article, the automatic void generation module of the new GEOUNED conversion tool from CAD to MCNP geometry is presented. The module combines common approaches, such as the division of void space into void cells, with two new features: fusion of void cells and independent void regions definition with individual fill material assignation. In order to evaluate the performance of the module and the capabilities of the new features, the CAD model of the IFMIF-DONES Accelerator Systems (AS) is converted with different void mode configurations. The best configurations are determined comparing the computational performance and the void quality of the converted models.