Is there a cubic spline source block in Simulink?

Hi

I am solving an optimal control using fmincon in conjunction with Simulink.

I am using a Repeating Sequence block as the discretized input and then fmincon chooses the best parameters for the sequence.

I am having convergence issues and have done research and people often use a cubic spline to parametize the input. The linear interpolated method the Repeating Sequence block can make thigns non-smooth.

My question is, how can I have a cubic spline source block in Simulink? What is the best way to do this?