Add a fillet and radius command. Allow the user to click either a fillet or radius tool, enter the dimension of the fillet or radius, select a polyline, polygon or rectangle, at the closest control point to the selection add additional control points and form the fillet or radius. If the user selects a rectangle, convert it to a polygon. Include an option for the user to change the coarseness of the radius / number of added control points (unless you have a way to make it a true radius).