No, for voice mail you can send it to an extension that has been externally forwarded, but not in DND. From the auto attendant, you could use a selector code to get to the extension, but you could also dial the extension number directly. From a mailbox, you can *8 and dial the extension number that is forwarded, OR, if you have the personal operator option, you could set your personal operator as the extension that is forwarded.
The best senario might be to have the auto attendant answer first, give the user's extension number, their mailbox answers and says to leave a message, but if it is an emergency press 0 which will send them to the forwarded extension and the outside number.