Hold on. Do we agree on the following: An order `k` B-spline is a `k` times continuously differentiable piecewise polynomial in the Bernstein basis. Converse is not true, not every piecewise polynomial is a B-spline. 

So, I'd see it a construction issue, not conversion: construct a polynomial having a given set of derivatives at breakpoints. (This is actually done, am sending a PR.)