If the drive and volume are visible to OSX, then it can be added to Auto Import, however...
Configuration of MC Auto Import folders requires that you Browse to the location, which must already exist, and then select it. In Windows, if you type in a non-existing location instead of using Browse when Adding an Auto Import folder, you get a message saying it doesn't exist. I don't know if you get the same message on a Mac.
What does this mean?
I also cannot add this drive to the Main Library.
How are you trying to add a drive to the Main Library, if not via Auto Import?
Regardless, if Handheld Sync works in the Mac version, which I don't use so I don't know if it does, then Handheld Sync is a better solution than your idea. You can just plug in your drive, set up Handheld Sync to select which files to transfer, what to convert them to, where to place them, what to rename the files to if you wish, whether to include Playlists and what format to use for them, whether to transfer Cover Art files or create them on the drive, what file type to transfer... you get the idea. It also means you can sync new files to the drive with special handling, or update files, or remove files.
So, have a look at Handheld Sync and see if that would work better.