Outlook 2000 includes an option that automatically adds a recipient's address to the Contacts folder when you reply to a message. Many users appreciate this feature because it helps them maintain contact addresses without requiring them to manually enter the addresses into the Contacts folder. However, this capability is only available if you use Outlook 2000 in Internet Only mode; it's not included in Outlook 2000's Corporate/Workgroup mode or in Outlook 2002.
In Outlook 2000, you can use a custom rule action and a free download to automatically add reply addresses to your Personal Address Book. You can get the AddToPab.exe package from Microsoft's TechNet Web site, but it isn't compatible with Outlook 2002.
Other add-ons provide this capability for both Outlook 2000 and 2002. For example, ExLife and CaBook, both from ORNIC, give users the capability to automatically add reply addresses to a Contacts folder.
You could also create a custom script that copies the addresses from the outgoing message to the desired folder, and create a rule that fires that script for messages that fit the rule's conditions.