Constructing a smooth closed 3D curve using splines

I need to create smooth, closed 3d curves that have zero curvature on certain given points. I am using the cscvn function from the Curve Fitting Toolbox to generate a curve passing by specific points. I would like to know if there is a way to enforce zero curvature on some of the points used as control points by cscvn.

I was thinking I could use different fittings for different sections of the curve and connect them later but, given that cscvn is already doing that, is there a way I can control the order of the polinomial it uses to make sure is just linear where I need zero curvature?