To do this:
1) Copy the Resource and Reservations Database to a local area or area reachable by file share.
2) Export the NME directory to TSV and isolate the rooms you want to migrate.
3) Modify the field for "MailFilePath" in excel its column "L", you can use local paths i.e. c:\nme\rnr.nsf or Network Paths i.e. \\machine\path
4) Save and import the file.
5) Run Locate notes data stores using the collection of rooms to be migrated, select "Find new data stores..." Add to Existing data, data to locate - Mail files - Hit next.
6) You should see on the next screen a 3rd radio button "Use MailFilePath from database", finish up the wizard and let it run, once finished you should see the updated info showing the server location and the file system address for the rooms.
7) Now you can migrate, pick your options and make sure to select "Through the file system" on the "Specify How to Migrate Notes Mail Files" screen.
8) Migrate as normal and it will pull the data through the file system.
