When the CAD program is able to generate a triangulated surface and export it as a NASTRAN or PATRAN file, save the assembly to a single file if the inter-region boundaries are common or shared or have the same perimeter triangulation. When this file is imported into STAR-CCM+, the boundaries can then be separated out into interface and non-interface groups and duplicate copies made of the boundaries at the inter-region interface. New regions can then be created thereafter and a single mesh continuum used to volume mesh the entire geometry resulting in a conformal mesh at the boundary interface.
If the assembly contains duplicate inter-region boundaries with different triangulation on each, then save each part to a separate file if possible. When you import these files into STAR-CCM+ you will get the choice of whether to create one region per file (recommended) or one region for all files. Regardless of the choice made, you will eventually end up with individual regions with triangulation at the inter-region boundaries being different. For the most part STAR-CCM+ can deal with the different inter-region boundary triangulation by using the edge zipper to match the perimeter triangulation and then deleting one of the boundary pairs so that a conformal mesh interface can be created between regions. However, depending on the variation in perimeter triangulation, the edge zipping may fail and you will have to use a different mesh continua for each region, resulting in a non-conformal volume mesh interface.