One question...

Theme: Line, drawing

How can I convert a polyline into a copper track?
First change the polyline so that it's layer is a copper layer. Then select the polyline by clicking a (here blue) line segment at this position (arrow) twice slowly in succession. This will select the entire polyline (more&more) and you will get three small black boxes at the line segment you hit:

1 (Fig. 1: Context menu)

Now select the item "Convert lines
into signal tracks..." in the context menu of a black box. TARGET then asks for the desired signal name for the track segments. Done.