Sorry, currently not possible. It has been requested a couple of times now but I don't really understand how useful it would be - at least if it's only converting a corona material to VrayMtl or Arch&Design... what about Light materials, renderer specific maps, lights, proxies, etc. It would be necessary to completely reverse the converter - for every renderer.
So in my opinion it doesn't make much sense to just convert one material class to another (CoronaMtl > VrayMtl or Arch&Design), but I just don't have the time (and the will) to do a full reverse conversion for Vray and MR.