I don't know how accurate you need to be but a very good result can be achieved by doing the following:
Using the offset command, offset the spline say 1".
Move the resulting spline off to the side.
Repeat the offset 1" but in the opposite direction (this will result in a "copy" of the original spline but will include control points along the spline).
Type "List" at the command line to see the length and a list of control point coordinates to replicate the spline using straight line segments.
No information on whether this is a one time or production situation. If production, just run a test side a little long and trim the excess off - measure the length of scrap. If a one time, well, you're already done.