Hmmm... I suggest performing technique, such as Nikyo, and slowly and gently applying the pin. The belt may tap too soon, but with time it should become quite stretched out!
Ha ha, couldn't resist. Actually, I have no idea about stretching a belt. Many assert that it is designed to not stretch. Maybe if it was damp it would have a little give to it, and you could repeatedly try stretching it? Good luck,