With the Exchange/Outlook combination, there are really only 2 options avaiable for creating distribution lists.
From Exchange: The Exchange Administrator can create a Distribution List that is available as part of the Global Address List (GAL). However, the Distribution List will only accept addresses that exist in the GAL. Therefore you would need to create a Custom Recipient for each outside addressee you want to include.
From Outlook: From within the Address Book, a user can create a "personal" Distribution List. Click the New Entry icon and choose New Distibution List. The user can then add addressees from previously created Contacts (by clicking the "Add Members..." button, or enter addresses directly using the "Add New..." button. If more than one user needs access to this Distribution List, it can be exported as a .pst file and imported by other users. Caution: in order for other users to use this imported Distribution List, all addresses must have been added using the "Add New..." option, or the users must have the exact same Contact list as the user creating the Distribution List if the "Add Members..." option is used.
Whichever route you go, someone is going to have to go through the trouble of entering in all those addresses. It's just going to be a matter of who - you as the Exchange Administrator (for the GAL Distribution List, or someone in the marketing department (for the Outlook Distribution List) (or maybe you anyway).
Hope this helps.