Display From field when composing a new message, Outlook 2003

[Danno]

Hello,

If a user A has Send As permissions to to user B's mailbox, and user B's
mailbox is opened as an additional mailbox in user A's Outlook profile - then
replying to messages in user B's inbox are sent as user B.

However, if user A wants to compose a new message and send as user B, he is
unable to do so because the From field does not display nor is it available
from the View menu.

Unfortunately, replying to an existing email in user B's Inbox and deleting
the contents to get around this problem is not an option as it is not time
efficient.

How do I get the From field to display for new messages in Outlook 2003?
What would prevent it from displaying?

Thanks,

Dan