Connect to backup drive to the laptop (I assume it is USB) and import all the files. iTunes will with the correct setting not copy the files to the laptop, it will just show that they can be played. This means that they will show up in iTunes even if you disconnect the backup drive, but it will fail if you try to play the music without the drive. Your laptop harddrive will not use any space for the music files!
But I think you should use my suggestion of setting up a second user (mine is called 'iPhone Admin') on your desktop and simply have both sets of files there where they can be backed up.
Is there a reason that you need the files on the laptop other than moving them to the iPhone? The second user with the compressed files works fine (which is what I do) and that makes it easier to generate the compressed version when you add new music to the library. My desktop is my only computer.