KiCad tracks do not support tapering. So you will need to use either some form of graphical polygon made in pcb_new or use one inside the footprint. The latter allows you to convert this tapered shape into a complex pad which can be assigned to a net unlike a graphical polygon.
Another option is the use of a copper zone but there you run into trouble if you place something near it.
However both would require you to get the dxf import running. If you can import it to freecad then you could use Kicad StepUp: The Sketcher for Footprint generation as a go between.