easy, just copy over the itunes music folder to your new hard drive, and then delete the original on the internal hard drive. then make an alias of the new location where itunes used to be, so itunes finds the music still. no need to import any mp3s or anything like that. alternatively, move the folder and go to itunes prefs, and specify where you put the folder.
sorry it isn't a great explanation, but i hope it helps.