Doing this appears to be in compliance with any Terms of Use, as long as you do not distribute the videos to others, and you retain any copyright notices on the files.
There are two excellent free services that take porn videos, download them to your hard drive, convert them to MP4 format and add them to iTunes. For Windows users, try Sex Vids on Pod, available here. Mac users should use PodTube, although it requires the use of Safari.