Are taller carbon fork expander plugs available?
The critical thing here is how much steerer tube length the stem is clamping. The expander plug does not add any structural strength and the cap must be of slightly less diameter than the steerer otherwise it would prevent the stem clamping it. 

If the stem overhangs the end of the steerer tub by more than a few mm, then the stem is mounted to high and could possible stress the steerer tube too much.

If you need to raise the bar consider flipping the stem so it’s angled up rather than down.