Since there isn't an exact guide for the inquiry you have posted above regarding Riverbed Modeler, I suggest downloading the PDF file from their official website because there is a topic called Building a topology and from what I've seen, it appears that it can be somehow used for your needs. The developers added some links to the end of the file that will take you to their support and information pages.
To download the PDF, click on this link and save the file to your PC. Your browser should be capable of opening PDF files, but if it doesn't, then install a PDF reader from this list.