You may charge at any kicks during open play, and conversions, but not during penalty kicks to touch or at goal.
if you indeed charge down a conversion, and that does happen, the only thing that will happen is that they wont get the extra 2 points, and play will continue with a kick-off.
if you charge down a kick at open play, you are more than welcome to catch it and run to the line. Keep in mind that as soon as the ball hits you everyone from the opponent is ONSIDE...
so let's say, u tried to charge an FH kick, it touches ur hand, as u dived for it, but then, it still travels 20metres behind u.. well, anyone from the other team who was 20m offside when the ball was kicked is now onside.. and can make the run to the line if he's smart enough to do that...
Hopefully I've explained it pretty much.. oh, I'm a certified ref btw..