
The trouble with trying to repair fabrics like this (i.e. fleecy or thin fabrics) is that, in order to pull the two sides of the tear together sufficiently firmly to make a good repair, the fabric ends up puckering. Powershield might resist this a bit, due to the woven face fabric. I'd probably start by bar-tacking the ends of the tear to stop it going further, and then try hand-sewn zig-zag. I'm not sure how successful you'll be.
And, of course, the knee is the worst possible place to tear it, as that's the bit that will stretch most, and rub the most. Couldn't you have torn them somewhere else? ;-)
Sadly, neither Point North nor Pennine sell Powershield, and aren't likely to.
Sorry, not been too helpful.
How big is the tear, BTW? If it's only small (say 1"), I'd just stitch it up fairly tightly, as the puckering won't be too bad on something this small.