It would mean completely stripping the saddle back to the tree and rebuilding the front once the knee pads were put on (why Americans all call them ploys I'll never know, poly refers to the lack of a horn on an Australian saddle). Assuming the front of the tree was big enough to attach the knee pads securely they could probably do it; actually I think I have seen some really old ones that had the knee pads attached to the leather rather than the tree so it could be done that way to I guess. Additionally, most Australian saddles have a fair bit bigger, or more pronounced, pommel than the average English saddle and part of that is because of the knee pads, but if the pommel isn’t there it could make it harder to do. Anyway, assuming you could find a saddler to do it I wouldn’t mind betting that it would probably be cheaper in the long run to just buy an Australian saddle.
I just had an idea, I ride in a wade saddle without bucking rolls, and whenever I'm getting on a horse I think might buck a bit I get a saddle blanket, not a pad, and fold it then roll it up so I had a kind of thick long roll. I would then tie it to the saddle strings at the front of the saddle with it over the seat behind the fork, so that when I got on I'd have the rolled up blanket between me and the fork and down the sides towards the front, (I think that makes sense), it would act like a make shift pair of bucking rolls. You could probably tie something like that across the front of your English saddle.