Actually, it's way easier than that, now! If you update to the latest 3.1 patch, original year is now an included tag on the second page in the tag editor.
For future reference, though, virtual tags only work with tags already configured in MusicBee. What you want is a custom tag, and you only need to define those if they are txxx/ style tags. If you go back to the main preferences window, you can find a list of possible custom tags already configured for you, in the boxes where it says "save to MusicBee database only". Changing that box to any of the included tags will cause your files to be rescanned for that tag, and new values to be saved in the file.