I'd like to gain some height on my handle bars, but currently my steerer tube is just proud (1-2mm) of the top of the stem (40mm stack height). I've read that the top of the steerer tube needs to be at minimum, level with the top bolt of a 2-bolt stem. Now that was for aluminum or a chromo-fork. That could be different for a carbon steerer tube because of the location of compression plug used to connect the top cap and the steerer tube.

My question is can I add a spacer under my stem to gain a little height?

See pic.