i've got a godawful kludge on one of my bikes that accomplishes what you want. but i'm not really recommending it...

i cut about 2cm of carbon steerer off a fork where there was some excess. i then removed the expansion plug from the steerer that needed the extra length, held the 2cm piece on top and inserted and tightened the expansion plug. this gave me another 2cm. the stem's upper pinch bolt clamping area covered about half of the extra steerer. like i say this isn't for everybody, but with that expansion plug in there acting as a metal sleeve connecting the two pieces i don't worry. some would. the expansion plug i used is quite long. almost twice the length of most others i have used.

although in your case, where you have a stem that works with the steerer, as is, i don't think i would mess with it.