I don't think that it can be done "easily".
You could try one of the following:
1) Use Out of Office Assistant and give up on the idea (not to say that they
won't add it in the future).
2) Turn off the Out of Office Assistant and create a rule using the "Reply
with a specific template" action and leave Outlook running while away.
3) Turn off the Out of Office Assistant and create a rule using the "Have
server reply using a specific message" action if you are using an Exchange
Server (new in Outlook 2000).