There isnt really a way to acheieve what you want without either possibly loosing some of the metadata, or resorting to an unsupported hack. See my response to Move some iTunes library items to different drive?:
The 'supported' way
You can disable iTunes from copying music to the iTunes folder for future imports by going iTunes > Preferences > Advanced > Uncheck "Copy files to iTunes Music folder when adding to library". This will add all future files to the library, but it will not copy them into the iTunes folder, so make sure you do not delete the files after importing.
The downside to this is that iTunes will not be able to keep those folders organized, so you would have to put them into the folder structure you want BEFORE importing them.
The hacky way
After importing the items into your library, move them to your alternate location, then create a symbolic link to the new location of the file. You have to use terminal for this:
ln -s /new/file/location/Britney\ Spears ~/Music/iTunes/Britney\ Spears
Assuming you moved the Britney Spears to
\ is used to escape the space in
Britney Spears. This is important.
I have not tested this myself, so I do not actually know if it will work. I will try it out later and update this post
After further pondering, I have hypothesised a modification of the first method that should let you keep your ratings. After unchecking "Copy files to iTunes Music folder when adding to library", move the songs with the lower ratings to the external drive. Now, when you try to play the moved tracks iTunes should complain about the missing tracks. When it does, point it to the files that you moved on to the external drive.
If this works, it would be a tedious, song-by-song process which may be impracticable for a large amount of songs.