Using Spline on the LEGACY MOD menu, you can change the shape of an existing spline by manipulating the points through which it passes (spline points). When you choose Spline, the SPLINEPNTS menu appears.

You can add interpolation points to a spline by selecting locations with the mouse. You can also extend a spline by adding an endpoint at a selected datum point or vertex, or by adding an endpoint on a second curve or edge.

Using Move, you can move an endpoint to a second curve or edge, or to a selected datum point or vertex, as well as move an interpolation point in a specified plane.