HT1449: iTunes for Mac: Moving your iTunes Media folderLearn about iTunes for Mac: Moving your iTunes Media folder
Currently Being ModeratedSep 14, 2012 7:24 PM (in response to pvolker)
You probably should have done:
iTunes: How to move [or copy] your music to a new computer [or another drive] - http://support.apple.com/kb/HT4527
Quick answer if you use iTunes' default preferences settings: Copy the entire iTunes folder (and in doing so all its subfolders and files) intact to the other drive. Open iTunes and immediately hold down the Option (alt) key (shift on Windows), then guide it to the new location of the library.
To answer your question, drag the media folder on the other drive to the Automatically Add To iTunes folder. It will add anything that's not in iTunes and ignore anything that is.