Add files to Home Videos without physically copying the file
iTunes menu > Preferences > Advanced tab > uncheck "Copy files to iTunes Media folder ..."

Note this will disable it for music and other media, too. If you want to keep using it for music and other media, disable the option, add all your videos, then re-enable it. iTunes will then keep track of the fact that the videos are in one place, and your music will still be copied into the iTunes Media folder when added in the future.

If you do this, do not select File > Library > Consolidate Files, or it will attempt to copy all your videos into the iTunes Media folder.