Another way to do it in 2008 is to add insert both figures into the drawing,
explode them, join them, and add the new polyline to the figure database (as
a new figure). Not perfect, but it may be faster than editing the field
book.
FYI, most of the Figure Line Editing Tool bar will work on survey figures
(be sure to update the survey database after the edits.)
Regards,
Peter Funk
Civil 3D Product Manager
Autodesk, Inc.