You have to do 2 things:
1. Map the fields in Desktop Manager. I forget the exact commands, but I think it's one of the advanced options under "address book" in sync options. That will allow you to map 4 Outlook fields to User1, User2, etc. on the Blackberry.
2. Change the field names on the Blackberry. Just open any contact, select "Options," "Edit" then scroll down to User[ ], select "Options" then "edit field name."
If you want to add/customize fields in Outlook, that will add another step.