I don't use ISA currently myself, so I'm not sure how it works.
But I did mess with it once, and I got my questions answeres at http://www.isaserver.org
Thomas Shinder, who is responsible for the site, is usually very good and fast in answering questions in his Message Boards.
He has a bunch of good articles and tutorials as well, which may cover your question.

Just in case nobody here knows the answer.