External Addresses in Exchange GAL

Can you keep and use external e-mail address lists in the exchange5.5 GAL. I do not connect the Exchange to the outside world – internal employees only, but use a pop mail system (MAILTRAQ) to retrieve and send external e-mail which is set up on each users outlook 2K with exchange and internet e-mail.
I need to globally distribute the companies external e-mail address list which is used via outlook to e-mail external clients.