Build order

while not a hot key what you want is already available. just select your desired next build and it automatically becomes the last item in your build queue. them drag it to the build next box and the current build item then is built 2nd. (this can be a pain however if you have already got a very long build queue since you have to drag the slider to get to the end of the build queue)

however to be consistent with how the tech tree build order works i would favor using ALT + pick build item to automatically put the selected build item 1st in the build queue