Sometimes its nice to pop a bubble. If you have to ask there isn't a snowball's chance in hades that you could do it - not that there is one good or sensible reason on the face of the earth you'd want to do it. To put it bluntly, its an idea that doesn't have a single up side and a whole lot of down side problems. Its not strong enough, its not ridged enough, its the wrong size, the brakes will be so inadequate as to be suicidal, none of the necessary accessories will have mounting points, the suspension system that goes with the frame will be inadequate in every way, oh, and do you have the ability to weld up your own exhaust system?
Now, could you do it? Sure, if you're a good enough welder and machinist, have frame jigs, all the materials on earth, and a great deal of knowledge of motorcycles, metal fabrication techniques, and considerable financial resources. Otherwise its just a suicidal pipe-dream. Anyone who could actually pull this off wouldn't do it, they would build a complete custom frame instead because it would be cheaper, easier to do, and the result would actually be useful.
