you picture isnt very clear..im not sure what you want exactly..can you try to find a better picture, one that looks more like what you want? Have you seen this efect in any other game?

That said, if you want to model clouds realistically, here are some general ideas..of course, in the real world clouds are indeed volumes, but they are also very irregular and partially transparent.. this is very difficult to achieve in 3d. You could use volumetric textures, but this probably not really a possibility in A6 right now, also they would be very slow.

The easiest way to make clouds is to make several spherical cloud sprites and arrange them in groups, this can look remarkably realistic if done correctly.

As far as somehow changing the lighting in the model to look like a cloud.. this is hard to say.. there are certain generally known lighting formulae...to do what you want, you have to search the various papers and shaders repositiories. Just do a search on the web for 'cloud shader' or something..you really need to know exactly what you want before you can ask for help on it