Hi, Tom
I don't have a Dreambox but it is possible apparently.
ITV HD is not a normal channel, but is an interactive stream, normally accessed by the red button on Freesat boxes.
The method to get it depends on what image you are using, for Gemini apparently you have to edit the
lamedb file, located in /var/tuxbox/config/enigma2
and paste these three lines in.
0f06:011a0000:090b:003b:134:0
ITV HD
p:BSkyB,c:000d49,c:030d49,c:040d4a,c:050001,f:5
Full details should be shown here:-
http://www.dreamboxuniverse.com/forum/index.php