Different materials for smoke How to apply diffuse, glossy or glass material to smoke in cycles without using volume materials?
This is not supported atm, it would require converting the smoke simulation to a mesh or some other structure (an SDF maybe). The smoke simulator can only feed Cycles 3d texture data, thus can only be represented as a volume.

Note that if you have access to Houdini or another tool with OpenVDB, you should be able to read Cycles OpenVDB smoke cache (new in 2.77), create a mesh from the cache, and export that back to Blender. But using Blender alone it is not possible to generate the correct data to use smoke sim ouput as a surface.