You won’t be able to do that. So, there are basically two ways of getting that kind of effect in a 3D program: either you add geometry or you “fake” detail by adjusting normals. Roblox currently only supports the geometry approach.
You could “bake” the cavity details into a texture map that can be imported to studio (which won’t look as convincing as it does in Blender), or you could just add geometry for those details to your model.