What software was the original layout done in?
There is a way to change lines into traces, but (I think) you have to go through an intermediate step using AutoCAD or equivalent software.
Look in the help for how to map layers in DXF:
One handy-dandy way is to export a board set up the same way you want to import stuff, to see how the layer names are handled.
In order to have a "real" trace in Layout you need to have a net connection between two terminals (pins) in a PCB Decal. These pin pairs form the basis of connectivity in Layout. Since 2D lines do not have that net connectivity they cannot be changed into traces. You can use the DXF file as a template. Import the DXF file, then add the correct PCB Decals and net connections then trace over the 2D lines with actual traces.
Or you might be able to cheat and convert your 2D-line to a copper shape.