Shape Stone doesn't allow one to destroy or create stone. So if the tunnel is ~2' round, then the stairs while stair shaped will still only be 2' tall.
The spell says stone or stone object, 10, +1cu/ft per level, so I think you can reshape the tunnel. The stone would just flow out into this hallway. That being said, at around 18 cu/ft a level we are going to need a bunch of of them to make a tunnel we can safely fight in.
OOC- I think passwall is what we are looking for.