How to use different mailboxes (sent folder) in Outlook for Mac
On the client-side, no.  

On the server-side, there are updates to Exchange 2007 and Exchange 2010 that address this.

If there is some reason that you can't update your Exchange server, then you could get around this on the client-side with a bit of work.  If the senders BCC the design@domain.com address when sending mail from that account, you could have a server-side rule on design@domain.com which automatically moves mail that is BCCed to design@domain.com to the Sent Items folder.  This isn't by any means a perfect solution, but it might work until you can update your Exchange server.