The short answer is you can't do what you think, easily. The simplest work-around is Al Morse's extension, (free. almorse.net) which encloses the positive. The 4/5 manual extension from Jake is worth the money, but won't give a third expressive division. The last solution is to use the CODM, or pay someone like Jake to enclose the Great for you. (I think it's something like jake@customhauptwerk.net, but can't remember exactly. Search the forum.....)
Tony.