You can register the CLSIDs associated with your custom DCOM interfaces with IIS by creating an in-process COM object that exposes the IADMEXT interface. This interface is discussed in Using Custom Extensions and IADMEXT Interface Reference. IISADMIN calls your interface methods and collects your CLSIDs, then stores them in the metabase where they will be available until IISADMIN terminates IIS services.
The DCOM CLSIDs stored in the metabase can be enumerated and called from other computers, so you should not register in-process COM interfaces.
You can find iadmext.h and other relevant header files in the \inetpub\iissamples\sdk\include directory.