If you have a backup of the files/folders (provided they are in the same format as the original) then open OE6, tools, maintenance, change store folder, browse to backup folder and say yes when it asks if you wish to use the new folder for your mail.
Sorry this may not be completely accurate as I'm trying to remember (I'm not on my home computer) but I've done this many times. Usually I create a folder called "mail" on another drive/partition. Then I move/copy the OE mail files to here (though OE will do this for you) and follow the above procedure. The benefit is that if I change my OS I am able to access all my old mail files in minutes of install.
Hope this helps
