yes and no. the fax line has to be its own DN. so you have 1111 is bobs voice mail 2222 is marys voice mail, and 3333 is the fax line.
You can install regular old callpilot client on top of your existing email for the voice mail, then create a new IMAP account on their email as well, the IMAP will be used for the fax. server, is the callpilot ip, user name is the extension (or 3333) and password, is the fax mailbox password. The imap should grab the faxes right up like any other email.