It is entirely possible to powerwalk while doing SS.
Wether it will speed up your loss, however, is another thing entirely.
Certainly in the first couple of weeks, I'd be suprised if you could keep up the powerwalking for long enough for it have an effect anyway.
And don't forget, to lose enough calories to shift a stone you'd have to walk from here to Egypt
so as good for you a walking is, it's not going to have a huge impact on your losses; thats why people who go to the gym but don't change their diets see slow losses - diet change is far more important for fat loss than exercise is.
Diets like LL and CD prove this, as you don't need to do any exercise at all to lose the weight, and as was said,
doing too much can actually cause the opposite effect and put your body into Starvation mode instead of Ketosis, which in turn will make it hold on to whatever bodyfat is has for as long as it can, which you really don't want!
At the end of the day, while doing a VLCD like CD SS, your body will lose what it is prepared to lose, and thats that, you just have to take it as it comes, safe in the knowledge that so long as you are 100% SS on it, you can't NOT lose the weight