I recently imported dxf files from a few different programs. They would export contour lines and/or elevation data points. Had to make sure elevation data was included in the export because the contour lines had no elevation data associated with them. You'd have to select each polyline, convert it, then assign an elevation to it, i think.
Although the imported contour lines were more detailed and precise, all I needed was the elevation data points to create and then modify the terrain.
P.S. each elevation point imports as crossed lines and a central elevation point. Double click too far from the center and the elevation data is not displayed. Also, they import as yellow, and i couldn't see them until dragging over an area.