I'm not sure i understand but you can insert points with AutCASD's Pedit. Then use CadTools command "Level out" on the line. Beware, level out smears out the elevation of all points based on the start and end elevations.
As an alternative you can draw a new 3Dpolyline, snapping against the first point (that has elevation) and then snap against any point or line without elevation. Finally you snapp the endpoint of your line against a 3D object with elevation.
Then use "Level out"