DJuxtaposition's position is flawed. The French construction savoir + infinitive is a far more general way of saying I/you/it can do something, rather than just meaning "know how to do sthg". However, assuming that you have eaten glass before, the English modal "can" could safely be dropped completely (cf. "Can you speak English?" "Vous parlez français?"). Straight "Je peux" is less common than you might think. The "cela", meanwhile, would only be taken as referring to the eating of glass, not knowing how to do so as suggested ("ceci" might do that, though).
However, in general French doesn't really go for writing two sentences with a semantic link without putting some kind of connector in there to make it patently bleeding obvious and somewhat wordier. Thus you'd be more likely to pass as a local with:
- Moi, je sais manger du verre sans que cela me fasse mal.
- Je mange du verre ; en effet, ca ne me fait pas mal.
- Manger du verre, oué, à moi ça n'me fait pas mal, hein?
- Je pourrais manger du verre, m'sieu, mais sans toutefois que ça me ferait le moindre ennui.
You might also wish to be aware that "verre" is a homophone for "ver", so there is a risk that people will think that you eat worms rather than glass. You must judge for yourself whether this is likely to reduce the effectiveness of the gambit.