One way to do this would be to use the built in Circle function present under Analysis: Fitting: Non Linear Implicit Curve Fit in Origin 9.1. However you would need to adjust the function to make it lie just under your curve. Also you would need to specify tangent points.
to find the radius of curvature at the maximum x value of your curve. (You can go to Analysis: Mathematics: Differentiate for single and double differentiation). If you wish to represent this visually, you could then create a circle object with the radius you found in your calculation. This is not the most optimum solution but it is one way of doing this.