I would suggest making the new terrain larger, then using the lower terrain tool to hide the terrain under what you have already made. I am not sure if there is an easy way to move terrains as the translate tool doesn't seem to have an effect.
I was able to move a terrain around no problem. I just adjusted the coordinates manually at the bottom of the screen to make it adjacent to my first one. This still isn't ideal and I hope there is a better way to simply change the size of an existing terrain though.
