From my experience the easiest way to import is by taking the old personal folders PST and doing the regular Outlook import to the Exchange folders. Imports everything exactly the way it shows up in the PST. Select import folders and subfolders into the same location in the Exchange mailbox.
Do you have a huge migration to do and that's why you don't want to do it that way? No Exmerge for 2007 that I've been able to find. Too bad too.