A sample ObjectARX application using the boundary representation library is included in the ObjectARX SDK in the objectarx\utils\brep\samples\brepsamp directory. This directory includes a Readme file that describes the sample application and how to use it.
The sample application is based on an interactive approach, which may not be the best approach for all applications. Objects are picked using selection sets, resbufs, and GS markers, and are transferred to the AcBr library using AcDbFullSubentPaths.