(presuming the external drive is apple/Fat formatted so that you can see it)
in iTunes preferences check the box under Advanced/General for "Copy files when adding" (and probably "keep organised" too)
then under the File menu choose "Add to Library" and choose the external.
Should copy them all into your iTunes > Music folder and organise them into folders.
edit: alternatively, copy all your music files into a folder (called "original" or something) inside your iTunes music folder on your mac, add them to the Library with "keep organised" checked. This has the advantage of moving all the files it adds into organised folders, but if there are any problems or files it skips or misses, they'll be left behind in your "originals" folder.