Here is some helpful information on multiple address books.
Yes it can support multiple directories, there are some limitations as to the type and data in the directories.
Are they just for viewing or email addresses? Domion support a Directory assistence db that lets you select the addtional directories and what they are sued for , email or/and access. It will also support LDAP directories.
If this is a customer directory there are some views that are needed such as ($Users) view that you can copy from a current Domino Directory and modify to fit your customer directory.
I do this for customer and web access directories all the time.