The way folks have done this is by using simple text files which act as the necessary real file system objects MC needs.
If you have only a couple of albums, just manually import some text files, and then add your metadata.
If you have many albums, it is pretty trivial to convert, say, a spreadsheet of the data you have, into these files and have MC import them along with the metadata.
If you're not able to do this, send me the spreadsheet, and I'll package up the goods for you.