Launch iTunes and select the music tab in the left sidebar. Here's the trick, hold the Option key when you drag and drop the music on to the iTunes window from the external drive. (all at once or little by little your choice based on what I say next) DO NOT release to Option key until the music is in iTunes. If you have as much music as you claim this could take a little while to finish. Now what this does is creates a File Path to the music on the external drive so it will not copy to your Mac. Note: your External drive must be connected to listen to the music in iTunes. Hope that helps.
you could also just do the following:
1. open Preferences/Advanced, and make sure "Keep iTunes Media Folder Organized" and "Copy files to iTunes Media folder when adding to Library" are UN-checked.
2. File/Add to Library (or Command-O). navigate to the external drive and select the folder(s) you want to import. This will add the music to the iTunes Library, but should not copy anything to the MacBookAir
this method will allow you to avoid having to hold the Option key for several minutes as you drag/drop