Do all email addresses need to be Google Apps accounts if I switch to Google Apps?

"You don't have to use the email functionality of Google Apps. If all you want to use is the other applications (Docs, Calendar, etc) then just don't configure the email functionality.

If however you are asking if you can have email accounts on both Google Apps and your existing solution then no, not really. What you can do is set up forwarding in one direction or the other, so your email for most users stays on your existing solution but for that one user it can be forwarded to a gmail account. Or you can setup a Gmail account and get it to import existing emails from your existing solution via Pop3 or Imap."