Hi!
Join 3D polyline joins other polyline segments to the first selected 3Dpolyline so the direction of the resulting line should be same as the first selected.
There shouldn't be any duplicate points in the resulting line, if so there are two commands to fix that. Look into "Polyline Tools", "Remove duplicate points...." and the "Weed" command.
Sure that can be done with a command but it's a bit odd. Before a command is implemented it must be evaluated, first it must be a command that replace a common procedure in Autocad. Even better if it’s hard to achieve with Autocad at all. Things that relative easy can be done with Autocads general commands isn't suitable for CadTools. Adding extra vertices in intersections can be done with ordinary Autocad “breake line at point” or “Pedit” command with some Snap settings.
A command similar to your request has crossed my mind before, "Breake all" would breake polylines, lines at all intersecting points. I haven't plan doing it but one never knows.
/Lars