Can any class dual wield? No. Some classes can and some classes can't. (I don't know all of the exact details by heart, so I'll pass on a further explanation.)
But can you switch weapons and add/remove a shield mid combat? Absolutely! On my 80 Brigand, I sometimes have to steal aggro from an add when I'm instancing without a mezzer in order to buy the party recovery time. When that happens, I use my hotbar loaded shield and defensive stance for a quick combat change.
However I've found that hotbaring an off hand weapon doesn't work quite as well. When I try to load a one hand weapon, it automatically switches it into my main hand. To switch from a shield to an off hand weapon, I have to manually drag the icons around mid combat.