ITV HD is Freesat-exclusive for the foreseeable future, so you'll need a Freesat HD box as well as your Sky box.
When you want to watch ITV HD, you'll need to connect up the Freesat HD box to one of the satellite feeds that comes into your Sky HD box.
This will leave you with only one tuner of your Sky HD box getting a feed, so beware of losing any dual recordings you had planned on that at the same time you're using the Freesat box.
When you're finished watching ITV HD, simply reconnect the satellite feed into the back of your Sky HD box. You may need to reset the box (unplug at mains, wait for a couple of minutes then reconnect) as it can get a bit huffy about losing one of its signals.