AFAIK, You can only re make new 3d polyline and replace old one with altered modifications...
Perhaps you could try to find something better to avoid loosing possible data attachments to reference old entity (3d poly), or store them before you do replacement and then put stored data back to new one... But like I said, AFAIK, you can't add Vertex and keep old 3d poly handle - only thing I see is replace and remove which is wrong IMHO or perhaps IEHO (In Everyones Humble Opinions)...
Marko Ribar, d.i.a. (graduated engineer of architecture)