Howdy,
Microsoft has a writeup on how to do what you're looking for:
http://office.microsoft.com/en-us/outlook/HA101642161033.aspx#2I don't have a copy of Outlook available, so I'm not able to test that. The one thing I'm concerned about is that your IMAP sent folder may not show up until you've "subscribed" to that IMAP folder. Subscribing to folders should be an Outlook option (though I'm not sure where!).
So I'd try the above link first, and if you just aren't seeing the folder you want to use as your Sent folder, then dig around in the Outlook options and see if you can find how to "subscribe" to it first.
Have a good one,
-Eric