You can register any recipient that you already have in the address book as a sender. To do that, go into the address book (User Tools > System Settings > Admin Tools > Address Book Management) and select the recipient that you want to add as a sender. On one of the tabs (probably either then main tab or the Email tab) you can choose to make that recipient a sender. Once you do that, their name will appear on the list of senders to choose from.
---Ricoh - Multifunction Devices - Ricoh Aficio MP C4501