Using Direct Mount brakes on braze on frame
I don't believe there is one. I found one image on Google that appears to look like it would adapt a standard frame/fork to take direct mount brakes, but I wasn't able to find a link to any manufacturer. There is an exceedingly brief discussion in this forum post.
Because direct mount rim brakes are a niche item, I wouldn't hold out much hope for an adaptor. In fact, the link is mainly sarcastic commentary.