The reason you have to separate the rear triangle is the belt is one continuous loop. You can't break the belt to slip it around the chainstay, therefore the frame has to separate for the belt to go through. It is possible to convert a frame for belt use, but it isn't for the faint of heart. You could easily do it wrong and ruin an otherwise perfect frame. The best option, unless you are very mechanically inclined, is to either buy a belt specific frameset/bike, or have an lbs or framebuilder do the conversion.