A full terraforming system is not currently planned at launch, it is more of a stretch goal. However there will be a system for some underground construction. From the last Q&A:
"We do want at launch the ability to build subterranean stuff, it’s not a full terraforming system it’s a tunnel system that allows you to build out structures and stuff underneath your house." - [Video Source] and [Text Source]
Whether this will work to do what you have asked we do not yet know. However, there is a Developers Journal coming up on Housing and Dungeons (currently planned for November 23, 2015) which might provide some answers.
