In this Zbrush's tip I will show you an easy way to extrude a mesh from any 3D object. All you have to do is import your OBJ file in Zbrush, then subdivided (the amount of subdivision depends on each object) and go to "surface" > noise plugin and play with the different noises / scale / rotation / etc... After that, mask the noise and make that selection sharper (optional). After that, go to subtool and extrude it to get the new mesh! EASY! As a tip, I would recommend as well to go to "deformation" tab and relax/polish the mesh to get a smooth result.
A tutorial on getting started with a couple of the most powerful concetps in xpresso: Remapping values and Iterating.
You can find the final scene here: https://dl.dropboxusercontent.com/u/14537088/remapping_and_iterations.c4d