Outlook 2000 (Exchange & POP)

Our Outlook 2000 is configured for Exchange. My CEO wants to receive his POP mail through the same client (no problem), but also wants to send (big problem). In Outlook Express, there’s a pull down button to select “From” for multiple POP accounts. This isn’t the case in Outlook 2000 (while configured for “Corporate or Workgroup” mode). When I try to manually send from a POP account (POP address in the “From:” field), I get the “..no permission to send on behalf..”. Is there a way to send from a POP account while connected to an Exchange server through Outlook 2000?