You can, but from first-hand experience, I wouldn't advise it. Like any journalist working against deadlines, I drink instant coffee by the gallon. When fall rolls round I sometimes throw a little nutmeg, cinnamon, ginger and cloves in with my instant coffee granules if I'm in slightly less of a rush. It's fine, but it's very bitter. It's not the same as a coffee made by a barista. The cinnamon in particular can be pretty gritty. If you have time, I'd grind your spices as finely as possible, but you're much better off with a proper latte.