No shell command per se. But there are at least two ways in zbrush (and more by exporting and importing but I’ll only cover zbrush native), one requires a ton of work the other not so much, depending on the object.
Work intensive way: If you have walls that need thickness, you can retopologize them using zbrush zsphere retopology. Under the Topology tab when you’re done, you’ll find a “skin thickness” slider, this will create a shelled skin from your topology, thick or thin depending on the value there.
Then you can reproject higher level details to the new skinned mesh by appending it a subtool and using Project All techniques.
Easier way: Mask off the faces that you want to extrude into a thickness for instance the faces of the wall. Invert the mask. Hit Crease in the geometry palette. Then while masked, go to the deformation palette and extrude the faces out that you want extruded and use the offset command or even the inflate command to extrude them out. Unmask and you’ll see that you’ve created an extrusion with creased edges to retain it’s shape for further subdivision. Now you can use the "Close Holes command in the Geometry palette to close up the back. However, the close Holes command rarely works correctly and can cause alot of nasty exploding verts more times than it will solve your problem.
In either case, doing it outside of zbrush via exporting and importing is the best option, but…depends on what software you use how effective you are with it. I would personally just use goMax, shell modifier, back to zbrush and project details, be done in a couple of minutes tops after touching up.
Regarding your last question, Zbrush has a Flatten slider in Deformation Palette. If you set up your model in the Preview Viewport so that it’s unified with the center crosshair at the center of your model (you can test this by turning on “floor” in the working window then adjusting it to zero out in the Draw palette) then you can simply drag the Flatten slider to the left or right depending on what side you need to be gone, and you’re left with half of your model…but it’s not ideal because that geometry doesn’t dissappear but only gets flattened up to the side of the piece. It may be fine, it may be a problem up to the piece.
You can also mask off half of your model that you want to keep then Hide Pt in Visibility palette and Del Hidden in Geometry palette. But then you’re left with a big hole (do not want for printing), and again Close Holes isn’t probably going to solve that, making you wish you had done it in max, seperating stuff there, capping holes etc…then back to zbrush.
Look at the plugin GoMax it makes all of this stuff painless if you’re a Max user. You could be done in the time it took me to type this. With this, You can work on the lowest subdivision level in max and it retains the subd history when back to zbrush.