[whpalmer4]

I would use File->Back Up Database... on the Mac, and pick a good filename that will make it quite clear that this is your old database. Store it somewhere safe, yet convenient to access. Then I would set the Mac to sync from the Omni Sync Server (which will probably make another backup with a less recognizable name). Add a few actions on the iPad and make sure they sync over to the Mac, and vice versa. The Mac should now be a fully-functional member of the trio.

Whenever you want to go back and look at your old database, just double-click that backup file in the Finder. OmniFocus will open it in a new window, and you can look at it, copy projects or actions to your current database by drag and drop, and so on. OmniFocus is happy to show you multiple databases at once in this fashion. Just make sure you don't make changes to the wrong database. I would NOT use the File->Revert to Database Backup... command to review your old data, as that would (attempt to) inject your old database into your new system, and that is not what you want.